The [data explorer](http://data.stackexchange.com) lets you run queries against a sanitised version of the database. The database is [up to a week out of date](http://meta.stackoverflow.com/a/77176/187228), so it should be good enough for your purposes. [A basic count for superuser](http://data.stackexchange.com/superuser/query/165633/number-of-users) returns 221877 at the time of this post (7th February 2014).

For something more up-to-date, you could use the [user query](http://api.stackexchange.com/docs/user-types) on the [SE API](http://api.stackexchange.com/) with the [`total` filter](http://api.stackexchange.com/docs/filters):

http://api.stackexchange.com/users?filter=total&site=superuser.com

This should be *almost* real-time - the API caches a little, but it's about as accurate as you'll be able to find.