Board Thread:Support Requests - Getting Technical/@comment-769831-20150220194658/@comment-26009167-20150220200508

According to mw:Help:Magic Words, your best bet would be, though this will only count users who have performed an action in the last 30 days.

Unfortunately, as far as I can tell,  would not work for this purpose, as it will show the total number of users across wikia.