@Jesse68. Well, I'm sure this situation comes up a lot since sometimes people don't know exactly what their plans are. If you haven't already confirmed / accepted the reservation, you can always decline with a kind note to the guest that they are welcome to re-book once they know how many guests will be coming. Inform the guest that you have extra charges for additional people so it would be best for them to book once they know how many will be arriving.
If you would like to accept this guest's reservation otherwise, you could say something like this:
Hi "Jim" (or whatever the name is) I would be delighted to have you as my guest, but I will need you to book when you know exactly how many will be staying at my place. Since the price of the stay depends on the number of guests, I will decline your request for now, but please feel free to re-book once your plans are firm."
This way your calendar remains open for anyone else that wants to book and knows what their plans are.
Or, you can follow the steps I outlined previously.
More questions? Ask away!