If you don't get a quick response, and it is urgent, I advise you to contact Airbnb via Twitter : @AirbnbHelp
At hte same time, maybe send a message like : "Thank you for your reservation. Please note that this property accommodates a maximum of 6 (six) people, and under no circumstances can more people be accommodated. Thank you for your understanding"
It does sometimes happen that people who are newish to the platform don't fully understand how it works, and they make mistakes when booking
I hope this helps