The limitations are pretty much determined by how fast of a computer you are going to be designing your game/program to run on. It also depends a lot on how many models are going to be in the screen at once.
What really matters is total number of polygons on the screen combined (from all of your models currently in view).
So if you are going to be doing, say, a space shooter. You are going to want to keep polycount around the 200-300 range because there will be so many there at once. But if it is an RPG or fighting game, where there aren't many characters, you can easily get away with a thousand or so per character on a reasonably fast computer.
Sorry for the long response to such a simple question, but there's really no easy answer
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