u can use global.asax file for that like
void Application_Start(object sender, EventArgs e)
{
Application["Users"]=0;
}
void Session_Start(object sender, EventArgs e)
{
Session["suer"] =Convert.ToInt32( Application["Users"])+ 1;
Application["Users"] = Session["suer"];
}
then on page load
Response.Write(Session["suer"].ToString());