Sorry to post twice in a row but I finally found where I read about Carnival and 2 person max per room. It's on the GTY through Expedia and the like. "A maximum of two passengers can be accommodated in a guaranteed cabin." That scared me off thinking they had a lot of rooms that were only for 2.
ETA: Here is the layout photo for Cat 4a on Carnival Sunshine from Expedia. How are more than 2 people even supposed to sleep in there?
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ETA: Ok did some googling and apparently there is a bunk bed that fold out from the wall opposite the bed. One must use the bed to get into the bunk bed if it is configured as shown. There may also be a trundle bed that pulls out from under the bed and lays in the small space between the bed and the bathroom or in the hallway itself. There is no place to stow luggage. Honestly, it seems ok for college students or young singles to pack into a cabin.