@Edward11 Contact the guest ASAP and explain that they must have misread the listing, because there is an extra guest charge for every person after 2 people, and that it is important for insurance and other safety reasons that the guest count is accurate.
Tell them to input the correct number of guests or I think? you can also 'change' the reservation to the correct number of guests and then the guest has to accept it.
If they refuse, hopefully you have a local co host who can go to the property and first speak to them, and if they won't fix the reservation you will have to contact Airbnb for a neutral cancellation, do not cancel yourself!!!!! and then your local contact can go to the property and supervise their check out.
Try to keep things as low key and friendly as possible until/unless they refuse or become unresponse.