You can have 2 or at the most 3 in a room, but you can have many rooms if you want to.
My suggestion is, go here:
http://www.gamefun4u.nl/page9.html download the demo and in the level after the trainings mission, count the total amount of enemies you encounter. When playing, press TAB and you'll see the framerate in the upper left corner.
I've tried to place as many enemies as possible without compromising the framerate, but as you can see in the demo, you can have a good number of them as long as you don't place them all in the same room or outside.