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up to a week out of date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

<p>For something more up-to-date, you could use the <a href="http://api.stackexchange.com/docs/user-types">user query</a> on the <a href="http://api.stackexchange.com/">SE API</a> with the <a href="http://api.stackexchange.com/docs/filters"><code>total</code> filter</a>:</p>

<p><a href="http://api.stackexchange.com/users?filter=total&amp;site=superuser.com">http://api.stackexchange.com/users?filter=total&amp;site=superuser.com</a></p>

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