Yep your right 300 will be the limit for some units(prolly barbarian inf).So in a 4v4 custom/multi battle you could have 8*19 units @300 men =45600 men.Plus 8 gen units @31 (maybe on huge unit size) =46048 men.Have to hire some hardware off nasa to play it though.

Have a look at this shot and see if you can count themhttp://www.gamespot.com/pc/strategy/rometotalwar/screens.html?page=78