@Lara177
Hi - there are a few things you can do. First of all put it in your house rules - be very specific about the number of adults and children that can stay. Also say: no additional guests may stay unless agreed in advance with the property owner. Also refer to it in your property description. Right now I can’t see anything specific regarding this.
when someone books - you should use the platform to re-confirm the number of guests staying. Also say that if they wish to have extra guests they must agree this in advance with you. It’s a very strong hint that you are keeping an eye on things.
Also a little white lie does no harm..it looks like you manage the place remotely? But in your final directions email there is no harm in saying ‘We May be around that week and will drop by to say hello and check you have everything you need.
finally - only make available bed linen and towels for the number of people staying. Lock away all other linen.