Can you bring more than 2 cars per campsite? It there overflow parking?

It depends on the site. We were at 15, and it only fits one car. It'll say on recreation.gov if the site fits more then one vehicle.

At the double sites you can fit more than two I believe

If you are trying to camp without reservations getting two cars in is hard if it's in a busy season. When we camped there there was a party of two who most likely reserved their spots ahead of time. There is no overflow parking