create one table for all the users, you need to have way to identity each user by having unique field. for example user name field can be used as unique column for your table. when user register you can check whether given user name is already given or not from your existing data.
when you need to get user details you can query using the user name column, for example
select * from Userdetail where username ='user1'
above will give only the details of user1 even you have 1000s or users in your table. these are very basics of SQL and ASP.NET, you better read books/articles/tutorials on this topic.
Any way ASP.NET provide more secure and easy to configure membership mechanism which you can try. plese check below CP tutorial
ASP.NET Membership and Role Provider[
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