First user is Jeff Atwood, the co founder of the Stack Exchange sites. 

 - http://superuser.com/users/1

As for total number of users, all you can do is guesstimate - on [users/sorted by reputation](http://superuser.com/users?page=1&tab=reputation&filter=all) you get 9 rows, 4 columns per page, so 36 users, multiplied by 5172. That should give you a rough idea. 

Also, I don't think that page shows unregistered users, so in reality there should be a lot more.