If you are a hotel that sells two or more of the exact same room, then you will usually have that set up as one listing on Booking.com, with an inventory of two or more. This is the so-called multi-inventory or hotel set-up.
On the Booking.com calendar page, this is represented as “Rooms to sell”. This number must be at one.
If you have one apartment that you rent out as a whole, and it has three bedrooms, then you still set your inventory to one on Booking.com.
If you then decide to rent out every room in your apartment separately, then you will need to create listings for each of the rooms on Booking.com, with the inventory set to one.