That's because there is no hard number. If you are performing simple per-vertex lighting you can do way more than if you have some elaborate multi-textured and bump mapped polygons. Beyond that it will do you no good to know this limit because once you have your game running, other things will affect this. Just make your graphics how you like them and work on your game. If it turns out you have too many polys for the effects you are using and the other things going on, then reduce the poly count. Usually the more important factor is the number of draw calls you are making. The Xbox GPU is a beast and can shred through triangles, but if you are only sending one or two at a time, you will kill it shortly because of the overhead of draw calls. The more you batch together, the more you can draw.
