Not a silly question at all and one which the Great Houses are still asking themselves.
I am playing this scenario as close to canon as I can. So, I've done a lot of research which includes Operation Liberation II and the 1st Succession War SB.
[PLEASE HOLD WHILE I GET THAT INFO]
CAT35233.Historical Liberation of Terra II - page 114 - Erosion of the SLDF [several tables] - Fate of the SLDF
6 BattleMech Divisions (= 36 BattleMech regiments)
11 Infantry Divisions (= 33 BattleMech regiments)
16 Independent Regiments (= 16 BattleMech regiments)
12 Warships
So, in November 2784 the remnants of the SLDF pledged to protect the Department of Communications possessed a strength of 85 BattleMech regiments and 12 Warships which is quite impressive. Jumping forward to CAT35235. Historical First Succession War and what is presumably the ComStar Expeditionary Forces (CSEF) [my name based on the Divisions listed therein] circa March 2788 before Silver Shield is approved shows a much smaller force, see below.
CAT35235. Historical First Succession War - page 34 - Those Left Behind
1st Division (SLDF 151st RBMD) [6 ‘Mech regiments]
2nd Division (SLDF 231st BMD) [6 ‘Mech regiments]
3rd Division (SLDF 197th MID) [3 ‘Mech regiments]
4th Division (SLDF 184th MID) [3 ‘Mech regiments]
5th Division (SLDF 13th RID) [3 ‘Mech regiments]
6th Division (SLDF 326th BMD) [6 ‘Mech regiments]
7th Division (SLDF 287th BMD) [6 ‘Mech regiments]
8th Division (SLDF 4 Ind Reg) [4 ‘Mech regiments]
Now this doesn’t count the Terran opposition (SLDF 79th MID and 123rd MID = 6 ‘Mech Regiments) faced during the canon operation which would have 43 ‘Mech regiments (37 ComStar Loyalists plus 6 Rebels) on Terra circa June 2788. That is a drop of 42 ‘Mech regiments in less than 4 years but does make sense to me. If you look at my old fanon Field Manual – Terran Republic 2785 I contend that the SLDF is an old force (personnel wise) that desperately needs (younger) replacement (nearly 13 years from the start of the Amaris Coup to the liberation of Terra is a long time without normal recruitment). Given their age and perhaps some growing discontent (departure of Kerensky, the flagging state of the Hegemony, start of the Succession War, Blake’s new direction) a wave of retirements during this time would not be surprising. I’d expect Blake and Hayes to also consolidate loyal commands and isolate potential problems accounting for the conflict that erupts in South America. I’d also expect the same strategy to be applied to the Navy as well.
This gives the CSEF a reserve of 42 ‘Mech regiments (at least equipment wise) from which they can draw upon to rapidly restore any damaged units during the conquest of Terra which is a lot more than I was expecting. Plus, this doesn’t account for other forces (Terran regional powers, mercenaries, friendly militia, etc.) which may be available that could help an expanded Silver Shield.