Can the hotel booking API book several rooms at the same hotel in one reservation?

Yes, in most cases it can. A hotel booking API usually lets you search for more than one room at once, telling it how many guests are staying in each room, and it will come back with what is available and what each room costs so the whole party can be booked together.

When you send the booking, the API normally asks for a guest name for every room, because the hotel needs to know who is arriving in each one. What comes back varies by supplier. Some return a single reference that covers all the rooms, and others split the stay into separate references, one per room. Both are normal, and your booking screen should show whichever the supplier gives you so your staff can quote the right reference if they need to call the hotel.

There are limits worth knowing. Suppliers usually set a ceiling on how many rooms can go through in one request, and once a party is larger than that it becomes a group booking, which the hotel handles on its own terms rather than through the ordinary API. Cancellation rules can also apply to each room separately, so cancelling part of a party does not always work the way people expect. Rooms can sell out between the search and the booking as well, especially when you are asking for several at once.

Ask your supplier how many rooms they allow in a single reservation, and make sure your portal collects a guest name for each room before the booking is sent, so nothing is held up at the hotel on arrival.