Before a guest arrives, seek out information in the message thread about the group. Get the guest to book correctly from the start or correct their mistake as soon as possible.
After the reservation has been made, but before the guest has arrived, get the correction made, because it's much hard to do so later. Start by clarifying the number and type of guests in the message thread. Then ask the guest to put in the Change Reservation request. Or you can initiate it the Change Request. Enlist Airbnb Support staff if you can't get the online alteration tool to work.
If you discover the extra guests only upon check-in, call Airbnb Support on the phone together to correct the number as soon as possible. If you discover the extra guests later, but before they have checked out, still try to correct the number. Don't wait until the day of their departure!
If you discover there were extra guests only after check-out, changing the number would be tough to do, if not impossible. I would use the Airbnb thread to ask the guests about it. Be transparent and ask them after the fact about what you saw and ask them how many people stayed at the house. If they answer you honestly in the message thread, then you will have documentation and an easier time if you want to request extra funds in the Resolution Center. Since it seems it wasn't entirely clear in your listing about extra guest fees, I'd let this one go but do let your guests know the importance of getting the count correct when reservations are made or at least prior to arrival.
Our listing is a three-bedroom house. We make-ready the beds for all three bedrooms and allow the guests to choose which they want to use. Rarely do our guests use more beds that they need. So for parties of two, we often find only one bedroom used. The beds comfortably hold five people (Q, D, S) so we prefer five guests or less. We only charge extra if there are more than five guests, $10 per extra guest per night, and it's built into the reservation settings to do so. We don't often see people intentionally misrepresent the number of people in their party, but occasionally they do. More often it's an oversight or parent thinking their 2+ age child counts as an infant. As you know Airbnb doesn't count infants as guests so that can make a difference. Since we cater to families and we need to prepare appropriately to host them, it's extremely important to us that the reservation accurately reflects the number of adults, children, and infants. Since changes were made to the "Families Program" last summer and changes were also made to the online alteration tool, we've been struggling with that, especially if we need to make a Change Reservation request.