extra bed
The Spanish term 'cama supletoria' translates to 'extra bed' in English. This is a bed that can be added to a hotel room, for example, in case there are more people staying than there are beds. It is a commonly used term within the hospitality industry. People also use it in homes when they are hosting more guests than there are bedrooms. It is not typically a permanent piece of furniture, but usually a temporary or portable solution for accommodating extra sleepers.
I can ask for an extra bed for my son's room.
This sentence is stating that the speaker has the ability to request an additional bed for their son's room, possibly in a hotel or some other lodging situation.
The extra bed turned out to be very comfortable for our guest.
In this sentence, the speaker is expressing that the supplementary bed they had proved to be comfortable for the guest using it.
It is necessary for you to buy an extra bed for when the children have friends over.
This sentence states the need for purchasing an additional bed in anticipation of the children's friends visiting and presumably needing a place to sleep.