If you need to set/change the Maximum occupancy in a space, navigate to your
Menu > Settings > Services > Bookable services > Choose respective service (e.g. Stay) > Space categories > Select one of your space categories
You can read more information about them here
Set your Capacity to being the "normal" occupancy of the room.
Set the Extra capacity to being the "allowed" extra guest(s)
Your various distribution channels will show the addition of both as possible to be booked.
However, the price will be calculated with the Extra or Empty bed adjustment in your Rate set up.
Click below for more information on how to create a rate