In order to be able to sell private rooms per occupancy on Booking.com you need to contact them and ask them to enable the occupancy pricing feature in your Extranet.
How can I recognize if it was enabled or not?
If you see a person icon multiplied by the number of guests with Edit option beside it, you will now that the occupancy pricing feature is enabled.
If you click on Edit option you will see a pop up window. Choose the Custom option and turn on the occupancy pricing for the number of guests you are interested in.
Choose if the % or currency option and put the discount amount you want for each occupancy.
The result on Booking.com calendar will be like this:
The blue rate is the one which received the price from FrontDesk Master and then the other occupancies are calculated automatically according to your settings.