My understanding has always been that it is not a specific number, but more based around a "feel." So, for example, on a night like New Years Eve when everyone is cram-err- gathered together on Main Street, the actual number is lower than on a day like Fourth of July when everyone is much more spread out everywhere.
This is why Disney put soooo many different spots for hats and horns, as well as various countdown spots on NYE: in an effort to spread everyone out.
It also has to do with (as I understand it) the ability to get emergency vehicles/assistance in and out of the Park.
Add all of that up and it's not a specific number, but a general flow and feel that can close the Parks.
- Dreams