Some places are very strict and some are very lax. We've been to both. Private campgrounds have a tendency to enforce the "X" number of people per campsite, much more than State Parks do. For private campgrounds, they can monitor things a bit closer, and because they are private owned, can do pretty much whatever they want. It's an additional money maker for them to charge if you exceed the posted number per campsite, or to even allow them entrance.
If you are a family of 5, with 3 small kids, it's pretty hard for anyone at State Parks to tell you, you have to reserve another campsite to accommodate the 5th (or 3rd child) or charge you more. I've never heard of that happening.
Indiana State Parks do enforce the 1 vehicle with a camping unit per campsite though. No problems with a camper AND a tent. But come sundown, the DNR guys will come around and tell you, you have to park all but one of your vehicles somewhere else (they always have a parking area near the gate house around the bath houses). This they do enforce.