The Myth III team's Andrew Meggs responded to a question on how many units can be on screen at once:
As many as you can fit. :)A single ghol standing in an otherwise empty field is 500 polygons. As you add more, the number of polygons per ghol decreases according to a formula that takes into account the number of ghols, the screen area each ghol takes up, and the performance of your system.
Well, that should reassure those of us who got their last system upgrade around the time TFL came out. Check out Andrew's full post in the thread below.