This question was answered on our Q & A Wednesday show on August 9, 2017.
All cruise lines designate a maximum occupancy on each stateroom category. This maximum generally ranges from 1 to 4 guests, but can be more for special larger accommodations. The room occupancy also ties into the allocation of life boat spots available for each guest. Once the room maximum is reached, no other guests will be permitted to be berthed into that room. With a maxed-out room, even if the guests are infant children, they will not be able to be added to the room and must be booked into an additional room. On certain cruise ships, there may occasionally be exceptions made where an extra bed can be brought in a stateroom to berth an extra guest or in the case of an infant, a pack-n-play or crib. There are also instances where you may not be allowed to add guests even if your room capacity allows it. When a ship is nearing or at capacity, cruise lines can close the sailing to additional guests. This can be a temporary situation, where more inventory could be opened up, or in cases where it is closer to departure, this could remain in place until the ship sails.