Late answer for anyone reading it. There are two key things here: We would want to keep the number of frames as low as possible (the series is square of 2, 3, 4… x), to retain as much visual quality (read about texel density). Secondly, the number of frames also depends on what looks smooth for a particular animation. For any major change 8 frames are good enough. For smaller changes, you go lower. Its a subjective thing.