Twenty Thousand

First, I shouldn't have treated 20YU, TR:3050 and MW2e as representing a single point in time. I've rewritten my "79k" post to reflect how things change from one to the next.

I don't know if the "1000 'Mechs per entry" trick will tease any useful data out of TR:3055; I doubt it'll work for TR:3058. (Fortunately I have the Field Manuals to work from. What will be trickier is figuring out how to account for ultra-light 'Mechs.)

ComStar's Cache Under Hilton Head

The Warrior Trilogy says "ComStar has more BattleMechs hidden here than any of the Successor States can claim." Clearly more than Davion's 110 regiments (14k 'Mechs), but probably less than Davion+Steiner's 110+75 regiments (23.7k 'Mechs). TR:2750 has twenty 'Mech entries; at 1k apiece, that would make 20k 'Mechs--right in the middle.

The MW2e affiliation table implies that ComStar expends 1200 'Mechs/yr against the Clans. At that rate, the cache would last 16.67 years, which would seem like a natural fit for the Truce-End date in 3067. 

Except ComStar already pulled at least 8300 to 8900 'Mechs out of the cache between the end of the Fourth Succession War and the Battle of Tukayyid. So the cache runs dry no later than early 3062, more likely by 3060, and even earlier if Focht prepares a secondary cache on Tukayyid. [Edit, Aug 28: Or earlier yet if ComStar continued supplying 'Mechs to the Rasalhague Kungsarme. /Edit]

• They lost a battalion (36 'Mechs) in their fake Death Commando raid on the NAIS in the 4th Succession War.
• Brion's Legion had less than two battalions when they sought asylum with ComStar in the 4th Succession War, and ComStar built them up to two regiments. (Per FM:Mercs, they don't use command lances, so call it 162 'Mechs.)
• ComStar gave Theodore Kurita enough 'Mechs to form twelve Ghost Regiments (about 1536). 
• I don't know how many would be lost during training. If it was the equivalent of hunting bandits, they might've lost 200/55000ths (200 from the 2/36 chance of rolling bandit on MW1e's affiliation table, 55000 for the number of 'Mechs in 3025) of their force per year from 3030 to 3039 (i.e., another 50 'Mechs).
• The ComGuard began deploying in the Federated Suns and Draconis Combine in 3030. Other nations followed, with the Capellans last in 3038. By 3043 there were so many ComGuards that the office of Precentor Martial was created to manage them, and by 3050 the ComGuard fielded 50+ regiments (6400+ 'Mechs). 20YU also refers to the ComGuard as "exceeding 35 BattleMech regiments;" if their numbers increased at 320/yr from 3030 to 3050, they would have reached 35 regiments around the same time the office of Precentor Martial was created. 
• The ComGuard gained experience by fighting bandits and other non-state forces. At bandit-fighting rates from 3030 to 3052, they might've lost another 256 'Mechs. 
• Pirate activity (at least for the FRR) increased dramatically during 3045-3050. If the increase was universal, the ComGuard might've lost a further 102 'Mechs. 
• Per 20YU, the Rasalhague Kungsarme has "a few BattleMechs on loan from our Blessed Order." The number isn't specified. Aldis Industries is supplying them with assault tanks; the assault class makes up 10% of all 'Mechs, so maybe Aldis provides 10% of the Kungsarme's tanks, which maybe ComStar matches by providing 10% of the Kungsarme's non-merc 'Mechs (about 192)? 
• The Kungsarme's short history has been unusually violent. At 3025 rates of attrition, these ComStar loaners might have lost another 151 'Mechs from 3034-3050. 

The ComGuard's losses at Tukayyid and subsequent defections to the Word of Blake aren't defined well, but are more than covered by the 1200/yr. Interestingly, 1200 'Mechs is a little over nine regiments, and the 3055 ComStar sourcebook lists the ComGuard at a strength of 41 (i.e., 50 minus 9) BattleMech regiments.

Ulric's comments in Lost Destiny show Clan Wolf's 'Mech losses to be pretty similar to their human fatalities. The ComGuard suffered 40% fatalities, so perhaps their 'Mech losses were close to that too.