So far you've recieved a lot of good answers.
IMO, there is no real point where more shots causes a problem with accuracy.
There definitely is a point where more shots will make reloading almost impossible if your shooting real black powder.
For me, after I've fired the first shot, all of my shots hit what I'm aiming at (or darn close to it).
I'm not trying for that last 1/4" of accuracy so I don't wipe the bore between shots. I only wipe it when the fouling starts to make reloading difficult.
When this happens, I wipe the bore with a damp patch and then a dry patch before loading the next shot.
When I'm shooting Pyrodex, the fouling doesn't seem to accumulate so I don't bother with wiping. I just keep shooting.
Shooting Pyrodex, I've been able to reload and accuretly shoot well over 50 shots without wiping or cleaning the bore.
edit:
Noting that this post is in the Flintlock section I must point out that Pyrodex works VERY POORLY (if at all) in a flintlock.
If you use Pyrodex in a flintlock with real black powder priming in the pan you will be lucky to get one shot for every 3 flashes in the pan.