As many as I can. Ancient era wars are usually small, unless I'm trying some HOF game, then I'll have 100+ horsemen armies with me and even more spears. Medievel forces are usually 40-70. Knights, pikes mainly. When Cavs come around I will usually go past the 100 unit mark. Infantry usually numbre 100-150. Tanks are less because they can massacre infantry so its usually under 100. Then MA and MI are well over 150, because MIs in large cities are near impossible to defeat.
But it all depends on how big the map is and how powerful the enemy is. When I played HOF games, I had over 150 knights, and it would go well over 200 cavalry if I had continued. Those games get ridiculously large.