I have made missions with slightly over 900 ai units on it, and the impact isn't at all noticeable as long as you spread them out nicely, and if you have a commander role, make sure he doesnt move a lot of units at the same time, otherwise the server crashes.
another perhaps important factor to keeping client performance up is having a low number of different types of units (vehicles) and by extent different textures, that way the texture needs to be loaded only once, instead of having many different types of textures.