Carnival Corp has an internal policy that ALL cabins MUST be sold before a ship sails. This policy applies to the entire fleet of 101 cruise ships.
They just about always are fully sold out.
My ship has not had an unsold cabin in more than 17 months.
Sold does not always mean occupied.
Nearly every ship has a 1% no-show rate on nearly every cruise.
Depending on the size of the ship, that could translate from as few as 2 or 3 - or as many as 25 - sold but unoccupied cabins at sailing time.
Many cruisers who miss the ship on embark day may still have an opportunity to catch the ship later in the cruise. Sadly, most of them are not polite enough to contact us to let us know if they plan to join us or not. If we have not received confirmation of a cancellation, we cannot give the cabin away. But we can use it for emergencies until / when / if the occupants show up.