Hi Tom!
We learned really quickly that this was a common guest MO. We don't live on our property and its pretty secluded. We have a Ring doorbell that alerted us to the issue-- reservation for 2, but SURPRISE! Four people with bags coming in! Here are a few things we do now.
1. House rules include a note that extra guests will incur charges.
2. Per night fee for guests over a certain number
3. Note to guest before check in verifying that they still are planning on the same number staying as they were when they made the reservation (nicely worded "we understand plans change") If they have more guests, we send instructions on how up to update the reservation.
4. We took the sofa off the listing and mention of the air mattress-- we really only want 4 guests and the extra sleeping space was giving a different impression. We mention this to families or trot this out when needed, but it makes more sense.
So far so good with the above. Its really cut down on the extra people. Best of luck!