In addition to @timtrueman‘s answer of “The data is my precious! preeeeciousssss”, it’s hard to really count it. Do you just want a plain number of how many people visit the site? There are a lot of lurkers (or people who find the site via google, get their answer, and don’t bother signing up). You can see some data on quantcast about that.
Otherwise, you may want to look at the Awards page.
Just check out the kind of awards and how many people have gotten them to get a better idea of the size of the community. Sure, 18k people have written an answer on the site. But ~3k people have surpassed 100 lurve.
I’d like to think that, given the amount of hours I spend on this site, I count as at least 5 members.