First user is Jeff Atwood, the co founder of the Stack Exchange sites.
As for total number of users, all you can do is guesstimate - on users/sorted by reputation 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.
