ust for curious I wanted to know how many users are there in superuser.com and Who is the 1st user?
If I click on the users tab,it only shows me the new users details. But is it possible to know who is the 1st user and till now how many users are present.
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 The database is up to a week out of date, so it should be good enough for your purposes.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.
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.
You can go to the Stackexchange front door and list all sites sorted by the number of users. You'll find Superuser on rank #2 with 224k users. If you hover with your mouse over that number, it says right now 223,678 registered and unregistered users. (This number is higher than the number I got from the data explorer's query, so I suppose the number is more up-to-date.)
