@Nicky39 I always ask the guest the confirm the number of people in their group before accepting a reservation request. In your case, you can ask them to cancel the request and submit a new one with the correct number of guests. If they do not do this, you can decline with a kind note for them to resubmit their request with the actual number of guests.
Best that they cancel the request as opposed to your declining, but if you have to decline, then that's ok too.
You should know that after a reservation is confirmed, but not yet started, either you or the guest can make an alteration. With alterations, the other party has to accept in order for it to go into effect.
For the host:
https://www.airbnb.com/help/article/50/i-m-a-host--how-do-i-make-a-change-to-a-confirmed-reservation
For the guest:
https://www.airbnb.com/help/article/913/can-i-change-a-reservation-as-a-guest
Hope this helps!