[Edi]

You can have upwards of 200 units on one commander and probably more, but after 9 rows of 20 it gets really bloody annoying.

Not that you'll see that kind of numbers in-game, that was in the good old days when we still had a debug console to cheat with before Johan removed it from the game.

Once you manage to build up your forces some, maybe three rows of 20 is the max you can dedicate to a single army unless you're going to assault a heavily fortified position.