Each Private Room on Booking.com, i.e. Double, Triple, etc. can be also offered under so-called "Single Use Rate". The rate allows selling the room to a single person who doesn't mind staying in a bit larger room.
First, make sure that the Single Use is enabled in your Booking.com Extranet. It should be visible for you here if it is enabled:
Then, please enable this option in Channel Manager Settings:
Next, please make sure that the price for one person is set up either in Price Manager, either in Availability & Rate Calendar, depending on which pricing tool you are using.
Availability & Rate Calendar