If you are looking at the Dream or Fantasy,
this thread might help you out. I don't believe we have a similar thread for the Magic and Wonder, but there is a list that used to be available in older versions of the Passporter guide.
As others have mentioned, you will have a room with bed spaces at least equal to the number of people even if one is an infant. Every passenger is counted towards stateroom capacity and lifeboat capacity.
Enjoy your cruise!