Each room/apartment has a defined maximum capacity. Every person (whether an adult, a small child or an infant) is counted as a person. There are certain tourist standard regulations that specify a number of people allowed per an area unit (square metre) of a room or apartment. Sorry, there is no extra bed option. In other words, if it had been possible to add an extra bed, we would have definitely done it, to sleep e.g. five instead of just five persons.