If you are talking about an act that you could get disfellowshipped for, perhaps quite a few more than one would assume. Too many get drunk or abuse drugs. You hear about the pedophiles all the time on these boards, and there could be others setting up still others for this (there are, last I knew, more than 25,000 known pedophiles in the religion). And, there are probably quite a few that do fornication, or "loose conduct" (as they like calling it when you get "too close") and don't get caught. Apostasy aside, I would say that as much as 15-20% of all witlesses do it, and that could be as high as 50% or more among those who are baptized and under 30.
However, if you include "sins" for which you only get hounded, I would estimate something like 95% or even higher. This would include those that go to the bar once or twice a year but do not get drunk. It would also include those watching R-rated movies, listening to rap and/or heavy metal music, sending children to college, those who often miss boasting sessions or field circus because they want to get out of town, or that are "materialistic". Under 30, it jumps to near 100%, since masturbation and viewing porn is included. And, if you include viewing the Christmas lights on purpose (such as driving around to see them) or watching Christmas specials or listening to Christmas music on purpose, it could be even higher. I would expect very few under 70 to be totally "clean", and even those above 70 could be inciting "It wouldn't hurt to drive around and enjoy the Christmas lights" or "It isn't that bad to enjoy the Christmas carols if you are doing it for the music itself and not as part of the celebration".