Explain to the guests that your property is based on a per person for any guests over five and that if he had put in the correct number of guests staying when he booked he would have been charged correctly on booking . @Ikhtiyor0 say you are sorry that he has misunderstood the basis on which he made the booking .
have you sent the guest a request to amend the booking .
if he doesn't pay for the ninth person you can ask them to leave .
you are of course likely to get a less then sterling review but you're likely to get one anyway and you can leave an honest review