I'm not sure how you are setting up the unit part. But if you're doing it with buildings that train a unit, then if you use
event
player begins training a unit.
and then you do an if check to see how many units that player owns. if they are at or above the amount, cancel the training. Of course, you'll have to set this up exactly how you want it to fit your map, but you get it I hope. | 
