Typically, campervans will sleep either two or four people.
What makes the difference is the model of campervan, because some models will be two berth, and others will be four berths. ‘Berth’ is originally maritime terminology and simply just means how many people it sleeps.
Normally in a 4 berth, you’ll have one fixed bed inside the camper, and one bed on top of the roof, in what we call a ‘pop top’. A pop top is a compartment in the roof which extends to reveal a bed for two people.
Some campervans will be without a pop top, so they generally sleep two people, and the campers with pop-tops will sleep four.