I made a reservation and for the equipment i chose 1 large tent over 9X12' and there are 6 people in our group with each pair having their own tent. Does that mean we are only allowed one tent for the campsite?

This is allowed as long as you stay within your campsite.

You should be fine as long it is 6 people. Happy camping!

You should be fine, they care more about the number of people. You could always update the rangers when you check-in.