We haven't found much issues with enforcement of the number of people at a campsite in AZ or NM state parks. We have been to nearly every state park in either state and always with at least 6, more if the kids brought their friends, and never an issue.
We don't do private parks unless there are no other state, COE, county, fed, or military parks options, of those we never paid the extra person fees, they did not seem to care as there was so many kids running around the RV park I guess it was too difficult to keep track.
I concur with the statement that if your campsite is not bothering other campers rangers are not overly concerned. I don't waste my time worrying about what my camping neighbors are doing as long as they are not too raucous, just too busy having my own fun to give a hoot.