Web API for hotel booking

I am working on an API for booking hotel rooms to implement a business logic to for adding guests for existing booking :

Add another guest to an existing booking. The required input is: ● Booking Id ● First name ● Last name If the booking is in a German hotel, the guest’s title (Herr/Frau/Dr) is also a required input field. It is also important that there are beds for all guests. The number of beds differ depending on the room type. The room type is identified by a room type code (a string). The following room type codes are currently in use: SINGLE 1 bed DOUBLE 2 beds TWIN 2 beds TRIPLE 3 beds

Need a help to find business logic to this requirement.