You Can Use Session_Start to Increase number of users and
Session_End Event to decrease the number of user in Global.asax (Global.asax.vb) file in ASP.net
If you don't have this file in your project you can add it using Add New Item - Global Application Class
Here is sample Event for your Global.asax.vb file
Sub Session_Start(ByVal sender As Object, ByVal e As EventArgs)
End Sub
Sub Session_End(ByVal sender As Object, ByVal e As EventArgs)
End Sub