There are some "big sings" with forty or fifty singers where you can get the tingle factor, and there are some intimate ones where just half a dozen people bounce songs off each other all night. Both can be magic.

But somewhere between 15 and 25 singers (you get to sing every hour/90 minutes or so) is the optimum for me: you have enough singers to make the choruses really satisfying, but the group is still small enough for you to be able to interact with everyone else in the group.