Well Skipper -- the answer is obvious -- If you have that many stickers on the hull it's obviously time for a new boat. This is one of the easiest excuses to get past the wife "Honey, there's no more room for the registration stickers - time to go shopping"
Sorry -- couldn't resist (me bad).
As I've never experianced that dilemna, I'll leave it to someone who has gone through it to answer - but I would think that a razor near the lower levels (not the hull) and a lot of patience - because they are designed to flake off once applied - would do the trick. You might not get a perfectly flat surface, but one you can work with.
We'll see what the experts chime in with.