Ok fine i got the proper solution for this
void Application_Start(object sender, EventArgs e)
{
Application["OnlineVisitors"] = 0;
}
void Session_Start(object sender, EventArgs e)
{
Application.Lock();
Application["OnlineVisitors"] = (int)Application["OnlineVisitors"] + 1;
Application.UnLock();
}
<td style="color:Red">
<%=Application["OnlineVisitors"].ToString()%>
</td>