@Gigi76, given that they didn't let you know until after you accepted the booking that it will be 22 people, you are well within your right to charge for the extra guests. Explain to them that you accepted the booking based on 1 person and weren't aware that they were bringing 6 more than the maximum number. State that the maximum is normally 16 people, so they may either cancel the booking or agree to pay an additional fee for the extra people. Alternatively, as @Gillian19 said, you could contact Airbnb and ask them to cancel the booking based on the misleading number of guests.