Is there a restriction on how many people can go in at once?

Your whole group can go in

When I went groups were kept at 6 to 7

I saw a few groups around 6 - 8 people going in. There are a lot of tight places and areas where you will have to crouch and squeeze though, so it might be easier to break your big group up into smaller ones. 4 seemed to be the perfect size.

I saw large groups go through together.