@Eilish4 When are you finding out that there are more guests than have booked?
If it's before the check in date, go to the reservation in question and click Change reservation. You can amend the number of guests on the booking so it's correct. The guest will need to approve the change before it goes through.
If it's after the reservation begins, unfortunately this change feature no longer works. If you're happy to host the extra guests and you don't charge an extra guest fee, just leave the reservation as-is. If you do charge an extra guest fee, go in to the reservation and click Send or request money and send a request for the extra charges.
If you're having extra people show up and you don't want to host them, call Airbnb and ask them to rehome the guests. Guests should be booking for the correct number of people in their party. If it's consistently happening to you, though, maybe ask all guests to verify the number of people in their group as part of your welcome messages.