Not 100% sure but I believe it is preference really as to how many you want to do, how fast you want them to go by and how long each frame should last. But whatever preset or your sequence setting is, make sure the frames match into it. Like for example mine is 24 fps settings. So if I want 12 frames in a second then I need each picture to last 2 frames. If it was set in the beginning 30 fps and say I want 10 frames per second. Then I need to make it so each frame last 3 frames. That would make it look a bit more choppy/glitchy If that's a good way to describe it.
It just needs to make sure it all fits into the sequence settings you pick from the beginning. Then it's all personal preference on how you want it.
Hope this helps.