It depends on when you sail. The most crowded time I've ever sailed was Easter week, and while it was noticeable more crowded than my usual experience, it wasn't oppressive or uncomfortable. Another thing to bear in mind is that they do limit capacity, so it's not like they will fill every room on the ship with four passengers. Once they reach a certain capacity they will only allow doubles to be booked, which prevents them from having more children than the kid's clubs can handle.