All doubles must be from a different canon point, unless one with a certain canon point is dropped. Then you can canon-update/degrade them to that point.

There can be as many doubles as players want. If there are 10 Narutos walking around, it might get a little annoying and people might be tired of seeing the character around. If there are 3, though, that might be more acceptable. But if players don't care about seeing 10 of them, and they're all consistently played with, then go nuts.