This depends on your army list as a whole and what you plan to do with it, so I would say there is no magic number. Skink-heavy forces have certainly been done before, especially in themed ones, but that many is pretty extreme, I must say. Provided there aren't so many that they just get clogged up, I suppose all those skirmishers (assuming at least some will be) could get pretty tough and annoying to deal with for your opponent, but once they start forcing panic tests on you an army composed of this many Skinks will quite likely begin to fall apart, and fast.