This one has eaten away at me for some time. There are options to set the foce sizes in military and civilian placements. 30, 60 etc. And a force readiness that states how many of these are patrolling.
It's not exactly adhered to, so i'd relaly appreicate the code level explained as to how many groups are spawned. I get usually much more.

If I recall, the force readiness is how much of that group is patrolling while the rest is garrisoned (in buildings). So if you got 100 units with a force readiness of 75% then 25 of those units are going to be garrisoned in buildings (Simulating off duty). @HeroesandvillainsOS can probably clarify that better than me though unless I nailed it.