The data explorer lets you run queries against a sanitised version of the database. The database is up to a week out of date, so it should be good enough for your purposes. A basic count for superuser returns 221877 at the time of this post (7th February 2014).
For something more up-to-date, you could use the user query on the SE API with the total filter:
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.
