Hmm, an interesting thought about TCS and crewing.
The TCS revenue generation mechanism suggests that different government types have different abilities/limits to funding a war machine in war or peacetime.
Perhaps the same percentage could apply to how many of it's population it could convert per year OR support to naval crewing?
So a Rich pop 6 planet may be able to build ships 'above it's weight' but is chronically short of people to crew them, thus the ships have to be built a certain way.
The Traveller Police can say what they like about decanonizing - TCS was never replaced with anything. The reality is that TCS and such (Imperial Navy Handbook) is all we have to work with.
Figure a Max of 10% of your population going into your military (This is what the US DoD defines as Total Mobilization). I'd say no more than 1% will actually be on a ship. The other 9% will be spread out to Naval, System Defense Forces, Ground Forces, Nautical Force Command, COACC, and their respective training establishments.
That addresses how many people you can get into uniform. The other issue is how your training pipeline changes in wartime. It gets faster at the basic level, but slower at higher levels. Advanced training is either abbreviated or eliminated entirely.
And there is the issues of building up the training pipeline to take those civilians and turning them into military types. Most peacetime militaries are not structured to take in that many civilians - they will lack facilities, uniforms, housing, etc.
It would take a year or longer just to build up the pipeline - there are no facility fairies to wave a wand and have bases appear. Especially when a large portion of the training pipeline takes place at Depots - Another reason that they are such high value targets - take one out and not only have you destroyed a construction and logistical nexus, you have also wiped out a good portion of the training facilities in a sector.