@Corinna76 You don't even have to put it in writing, when guests break your rules you can call Airbnb and have them cancel the reservation penalty free. It is clear that there shouldn't be more people than the number of the reservation. But of course you are free to give more detailed description, maybe you don't want guests to have visitors at all, or if you just want to confirm the max number of people allowed, you should write it in your house rules. Although I don't know about the refund, if you cancel guests tend to get refunded for the unused nights.