Many games facilitate better performance by increasing/decreasing the number of triangles/polygons that are drawn, depending on how close the camera is to said object. Mountains, viewed from far away, could become literally flat. Models would steadily lose resolution as you walk further away from them, etc.
If I want to implement such a system, how many different permutations of the same model/models should I have? Suppose the model is 20,000 triangles when viewed up close, but then I halve that when the camera goes away, so now it is 10,000. Then again, when the camera is even further away, making it 5,000 triangles. Would three versions be enough? Or is this really just an arbitrary implementation? Does it depend on the game itself?