Hello @Lugene0,
When a guest makes a booking Airbnb calculates how much they pay by taking your room charge and multiplying by the number of nights. They also add any extra person charge by the number of nights and the cleaning fee, their own guest fee and any applicable taxes or currency conversion fees.
You could try going to their reservation and clicking on its "Send or request money" link to request an extra payment.
You could also try the following more official way but note that it will temporarily leave you open to requests for 7 guests:
increase your maximum to 7
set an extra person charge for any guest after 6
submit a reservation change request to that guest, setting the reservation to 7 guests
wait for their acceptance
Whether they accept it or not, reset your maximum to 6.
If they don't accept it the reservation stays as a reservation for 6.
As your maximum capacity is 6 guests you can insist that only 6 will be accommodated but you'll have to be on site to enforce it. If you have messages on the Airbnb system where the guest says that they intend to bring 7, not including infants, then you could contact Airbnb support and ask them to cancel the booking if you wish.
Steve.