Problem with visitor count

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ASP.NET

I want to know total visters of this application but the following code shows only 20 times then after that the count again starts from 1. What's wrong with it?
How should I count total visiters. :-\

Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
       ' Code that runs on application startup
       Application.Lock()
       Application("tusers") = 0
       Application.UnLock()
   End Sub

Sub Session_Start(ByVal sender As Object, ByVal e As EventArgs)
        ' Code that runs when a new session is started
        Dim tusers As Integer
        Application.Lock()
        tusers = Convert.ToInt32(Application("tusers").ToString)
        Application("tusers") = tusers + 1
        Application.UnLock()
    End Sub

[edit]
1) Title (Always use a meaningful name for your question - your username does not us give any idea what your problem is)
2) Use a "code block" around code fragments: It preserves formatting and makes it easier to read.
3) Grammar, punctuation and upper case (Don't use all lower case, it is considered "childish" like ALL UPPER CASE is considered "SHOUTING"
- OriginalGriff[/edit]