How Many BattleMechs Exist in 3025?

[Edit: more exact estimates on Aug 10, 2017 /Edit]

I estimate a ceiling of 54 thousand BattleMechs.

383 regiments employed directly by the Successor States
(plus 744 regimental and 1152 battalion command lances)
24.83 regiments known to operate in the Periphery
(plus 18 regimental and 22 battalion command lances)
3.2 regiments estimated among lesser, unnamed Bandit Kings
2.75 estimated merc regiments otherwise (un)employed
11.5 regiments estimated under corporate control
5 potential regiments of unlisted FWL provincial forces

[Edit, Sep 1, 2015: but see also this post. /Edit]

Such an exact accounting of forces might seem restrictive to some people. But like the Kurita house book says:

Although efforts have been made to gather the most up-to-date information possible on the forces, readers must remember that regiments are constantly being reformed, transferred, temporarily removed from active service, reactivated, or decommissioned for spares.

Especially with mercs, there's a constant churn of big units fragmenting into little units, and little units joining up to reform into big units, so there's plenty of room for a Player Unit of just about any size and type to form up in just about any part of the Successor States.

Details below the jump.

MW1e: Quick Plausibility Check

Is 26 years plausible for the typical length of a MechWarrior's career?

It's long enough to beget and train an heir, yet not so long that 400+ MechWarriors/year out of Sun Zhang becomes totally implausible. (Multiple factors would give the Combine higher turnover than expected.) So it's definitely the right ballpark, although pushing the long side.

Are 250 combat encounters (half 'Mech-vs-'Mech) per year plausible?

Yes, of course. It's a staple of the setting that 'Mechs often clash without being destroyed. Combining statistics given in BattleForce (1st edition) and the way MW1e awards XP should tell us just how little damage it takes to drive enemy 'Mechs off. (MW1e's Ceti Hussars entry implies that high intensity conflicts don't rise above 10% annual casualties.)

How big is a typical garrison?

Well, there's about 7 times as many 'Mech companies as there are planets in the original setting, but I want to look at 3025 proper. From the old House Steiner sourcebook:

• 42 regiments are assigned to the ~93 worlds within 2 jumps of Kuritan or Marik space. Assuming 12.5 regiments are engaged in assault or relief duty (Steiner's full 75.1 regiments * 4/36 assault & 2/36 relief), spreading the remainder as thinly as possible would give each border world a garrison of 34.2 'Mechs (minus however many have embarked on raids). 
• 16 regiments are assigned to the ~55 worlds within 2 jumps of the periphery. Spreading them as thinly as possible would give each of these outworlds a garrison of 31.4 'Mechs (minus however many have embarked on raids).
• 17 regiments garrison 13 key worlds of the interior.
• More than 140 interior worlds have no 'Mech regiments assigned to their region.
• "Garrison units usually consist of infantry, armored, and artillery regiments, whose personnel are mostly native to the planet. There are usually only a few 'Mech lances that pull garrison duty on a world unless that planet is strategically important."

Steiner could put a lance on every interior world if each of their 75 regiments detached two lances apiece. (This wouldn't break the setting.) However, the Trell system is only 45 light-years from the nearest Drac world, and it didn't get a lance until Oberon's raids became a problem. So the interior worlds probably don't have 'Mech garrisons either.

Page 30 of MW1e says "Unlike an assault... [raids] often involve a significantly smaller force (often of one to two Companies...)" I.e., 12-24 'Mechs is significantly smaller than an assault force. Since an average assault force is about 140% of the defender's mass (page 96), it follows that any world worth conquering will be garrisoned by significantly more than 17-34 'Mechs. The world of Chara III (also called Pacifica) has a battalion in garrison, though apparently that's due to its agriculture, rather than strategic positioning. Maybe it's not worth conquering.

So, based on my earlier figures...

• Key worlds average 1 full battalion, 2 full companies and 6 smaller units. A few add extra full regiments. 
• Theater Headquarter worlds are the same, except that 2/3rds of their forces are engaged in assaults, raids and relief. 
• Strategic worlds and the scant handful of other worlds within 1 jump of the Kurita or Marik borders have 2 full companies and 6 smaller units.
• Worlds in the Periphery theaters or 2 jumps from Kurita or Marik have 1-3 lances to fend off raids. 
• Worlds 3 or more jumps from the border are garrisoned by conventional regiments only.

A few incidental curiosities from the Lyran deployment table:

• CMO 26 (which I read as "Coventry Military Outpost #26") is on the map. CMO 1-25 either don't have 'Mech garrisons or are known by other names. 
• I'm guessing "Ellison IV" should be "Corridan IV."
• Two regiments in the cavanaugh theater are assigned to "Denebola V," which isn't actually in that theater (only three other worlds are). Don't know if the two regiments should be moved to other worlds in the same theater or moved to the other theater.Would have to check enemy deployments before deciding.

Final "What Were You Doing Last Year" Post, I Swear

Lots (tons) of unexpected time for reading in those autumn months.

• War of the Worlds (1898) by H.G. Wells: The archetype for giant robot warfare, and rightly so. Quick and easy to read, this should be required reading for any fan of BattleTech. (I also watched the 1953 movie. It was alright, and the parallels to Cruise's version were neat.)
• The Time Machine (1895) by H.G. Wells: I'm pretty sure that Willy Wonka's "oompa loompas" are, in fact, the "eloi" from Wells' book.  
• One Piece (1997-present): Joke-filled. Never fails to amuse. 
• Yotsuba&! (2003-current): A delightful manga about a small child being a small child. 
• Attack on Titan (2009-present): Super cliche story about humanity's last stronghold in a world overrun by monsters. Still manages to be fun.
• Deadman Wonderland (2007-2013): A fighting manga centered on super powered adolescents in a murder-themed theme park. Lots of cliches, but I'm enjoying it. 
• Vinland Saga (2005-present): A (seemingly) well-researched manga about Vikingdom. Features raiders and mercenaries in France, the conquest of England, and hints at an eventual move to Vinland. Half battles, half slice of life? The art and story remind me of Berserk.
• Uzumaki (1998-1999): A Lovecraftian saga of weird events in a small town. It starts as a series of loosely connected vignettes; I wish it had continued like that forever. 
• Sandman (1988-1996) by Neil Gaiman: A well-crafted plot, and mostly good art. I like how the plot of one issue is sometimes seen as background in another issue - usually subtly, though this is also done near the end as a major plot point. There's also an unfair tease at the end, and I'm not sure what exactly Gaiman intended with it.

MW1e: Personnel

MechWarriors see an average of 250 combat encounters (almost half are artillery bombardments, aerospace strafing runs, infantry or minefields)* annually. A career lasts 26 years on average, and about 1600 end (retirement, dispossession or death) each year.

There's 5/6ths as many technicians as active MechWarriors, and their military careers last about 11 years on average.

There's 1/6th as many scouts as active MechWarriors, and they last about 14 months.

Sphere-wide...
About 80 regiments are deployed as full regiments.
About 350 battalions are deployed as separate battalions.
About 600 companies (350 full strength, the rest 9-11 Mechs) deploy independently.
More than 1800 other groups of 8 or fewer 'Mechs are deployed independently.

That's almost 2900 deployments, about 2000 of which are defensive (25/36, using my trick with the events table). The Inner Sphere only has 1700 settled star systems! Too few for a single deployment per system, and too many for a single deployment per world (assuming better than 1.2 inhabited worlds per system). Best explanation, I think, is that relatively few worlds have 'Mechs in permanent garrison, and that each garrison consists of one large group plus multiple small groups, scattered across the world.

MW1e: Mech Production Rates

The old House Marik sourcebook says the Free Worlds League manufactures "an estimated 500 'Mechs per year," and the old House Liao sourcebook lists eight manufacturers totaling 400 'Mechs per year. The other three House books don't say how many 'Mechs they build.

However, from the Attack by Natives table I suspect that at least some tables in the first edition MechWarrior RPG (hereafter MW1e) are carefully non-arbitrary, and from my survey of Notable Pilots I know that numbers in MW1e can correlate systematically with numbers in other books. (Humans, writers especially, like to be consistent and will reuse numbers when they can.) In particular I want to look at the chart where players roll a d36 to choose their starting faction. Chances/36:
7 - Davion
6 - Kurita
5 - Steiner
5 - Marik
4 - Liao
2 - Bandit King
7 - Unaffiliated (i.e. e.g.,  periphery)

I doubt it's a coincidence that Marik gives you 5 chances and produces 5x100 'Mechs/year, or that Liao gives you 4 and produces 4x100 'Mechs/year. I think it's reasonable to take Davion production as 700 'Mechs/year, Kurita 600/year, and Steiner 500/year for a total of 2700 'Mechs produced annually across the whole Inner Sphere.

I also want to look at the "Standard Enemy Forces Table." It's easy to find the chance of any 'Mech turning up in a given lance type, and I've roughly calculated the chance of each lance type turning up during a "Battlefield Encounter."

If I take the ratio of Valkyries to other light 'Mechs encountered as also being the ratio of Valkyries to other light 'Mechs manufactured annually, and assume there are [30% of 2700] light 'Mechs produced annually, MW1e yields an annual production of 128.4 Valkyries. This is close to the 130 cited in TR:3025.

This vindicates my assumption about annual Davion, Kurita and Steiner production rates, and shows that the lance table is at least a reasonable guide for individual 'Mechs. It doesn't have every 'Mech on it, of course, so I'm hoping (for instance) that more precise calculations will push the Valkyrie entry up to 146/year to cover both Valkyrie and Spider production.

BattleForce says that 30% of all 'Mechs are light 'Mechs, but I'm reluctant to assume that same 30% also applies to annual production. Hopefully more precise calculations will let me discard that 30% weighting factor.

MW1e: Attack By Natives

The first edition MechWarrior RPG has a general weekly encounter table. You have a chance to be attacked by things, to receive reinforcements, and a number of other events, all being more or less likely depending on your current assignment.

There's no table to determine the players' current assignment, but you have to figure that most forces are in garrison, so the chances of getting some other kind of duty should correspond with the chance of an equivalent event happening to a garrison. If the Garrison Events column gives you a 3/36 chance of being attacked by natives, then I figure that's equivalent to a 3/36 chance of being assigned to Pacification Duty.

Now, whatever assignment you're on, if you roll the "attack by natives" event and get attacked while in your 'Mechs, you roll 2d6 (then modify the result depending on your assignment) to see what support they get. I wanted to know just how often the natives had 'Mech support.

I noticed something peculiar.

internet forums: be excellent to each other, and party on

I used to frequent a forum which had sometimes heavy-handed moderation. The rules there boil down to "don't be a dick." This axiom, known as "Wheaton's Law," was formulated by Mr. Wheaton for competitive environments where people all generally recognize (if not explicitly share) the same goals and social norms--things you simply can't assume to any degree of precision on an internet discussion board.

I think it's better to distill them into something adaptive and proactive:

Relax. Be friendly and helpful. Put each other at ease.