We post our limits, ask at the time or reservation how many are in the group and warn you that we don't make exceptions. We aren't stupid, we can count to seven.
We have a six person limit because it works for us. The facilities are sized for a certain number of people. Each additional person in a site adds to the usage of the facilities. One or two groups of 6 plus people can quickly overwhelm the restrooms. With only 1 RV there is only one toilet and sink for everyone in that RV, so naturally everyone else heads for the park's when it is occupied. Often, when returning to the park from the day's activities, everyone in the group needs to use the restroom for something, be it the toilet, brushing teeth, fixing hair, taking showers etc. That one group of 10 people would take up nearly entire restroom capacity of the park. Have two or three of those groups, and the other 97 reservations will be out of luck for hours at a time. Large groups just don't work in our parks.
OH, we also require an RV on each reserved site, so booking two sites doesn't work for us either. It isn't about the money, it's about the comfort of ALL of our guests, not the convenience of one group.