Are you able to pitch more than one tent on a single campsite or is it just one tent per site?

There are no limits on tents but no more than 6 people per site unless it is a double site. There are 2 double sites numbers 15 and 16 then you can have 12 people.

there's a limit on vehicles, I don't think there is a limit on tents. Some sites have quite a bit of space.