@Marco2, I don't understand your question very well.
Are you a host?
If you have a room and you want to put only one person in that room, you can refuse a second person if two persons arrive.
If you want to rent the room to 2 different persons or even more, like in a hostel, you have to make that very clear, otherwise you will have unhappy guests. But if you do, you need an extra listing for every bed you rent.
Exemple: you have a living room couch, sleeps two persons. You can rent it to a single guest or to a couple. You cannot rent it to two persons who do not know each other. You need one listing for that. You can set the price for the bed and a surcharge after the first guest, if you want that the second person pays an extra charge.
If you have two couches or 4 bunk beds, you need 2 listings for the two couches and 8 listings for the 4 beds with one guest sleeping bottom layer, the other sleeping top layer. As said, if you rent that way, it's like a youth hostel and it must be clearly indicated and cheap.
If you are a guest: make sure to know the situation before you book.