Most ships have a large number of three or four occupant cabins. On the next ship we will be on, the Royal Princess, approximately 50% of all cabins can hold at least three people, and of those, probably 30% can hold four people. If most of these, as you assume, would have as many occupants at they can hold, Royal would sail with almost 5,500 passengers, not the 3,560 she is listed as at "full" capacity. I rather doubt that having that many people on board would be enjoyable.