MultiUser For ASP.NET Web Form

use global.aspx file for that purpose like this:

Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
        Application("counter") = 0
    End Sub

Sub Session_Start(ByVal sender As Object, ByVal e As EventArgs)
        Dim cnt As Integer = Application("counter")
        cnt = cnt + 1
        If cnt > 5 Then
            MsgBox("hello kavita!!!")
            Response.End()
        End If
        Application("counter") = cnt
    End Sub

Sub Session_End(ByVal sender As Object, ByVal e As EventArgs)
        Dim cnt As Integer = Application("counter")
        cnt = cnt - 1
        Application("counter") = cnt
    End Sub

best of luk!