Count Online Active users

Then right click on your web project and then click add new Item -> then select the Global Application Class from template list window.
and Add the Following code

<%@ Application Language="C#" %>

<script runat="server">

    void Application_Start(object sender, EventArgs e) 
    {
        // Code that runs on application startup
        Application["Currentusers"]=0;
    }
    
    void Application_End(object sender, EventArgs e) 
    {
        //  Code that runs on application shutdown

    }
        
    void Application_Error(object sender, EventArgs e) 
    { 
        // Code that runs when an unhandled error occurs

    }

    void Session_Start(object sender, EventArgs e) 
    {
        // Code that runs when a new session is started
        Application.Lock();
        Application["Currentusers"] = (int)Application["Currentusers"] + 1;
        Application.UnLock();
    }

    void Session_End(object sender, EventArgs e) 
    {
        // Code that runs when a session ends. 
        // Note: The Session_End event is raised only when the sessionstate mode
        // is set to InProc in the Web.config file. If session mode is set to StateServer 
        // or SQLServer, the event is not raised.
        Application.Lock();
        if ((int)Application["Currentusers"] > 0)
        {
            Application["Currentusers"] = (int)Application["Currentusers"] - 1;
            Application.UnLock();


        }

    }
       
</script>

Add this Code in top of the Master page

Online Users:<%:Application["CurrentUsers"].ToString() %>