One new code

add a global asax file into your application then
create a new integer
in the void   Session_Start(Object sender, EventArgs e) even handler add this code
Application("Count",0);
in the void   Session_Start(Object sender, EventArgs e) handler add this code
int UserCount = Convert.ToInt32(Application.Get("Count").ToString());
UserCount +=1;
Application.Set("Count",UserCount);
in the in the void   Session_End(Object sender, EventArgs e) handler add this code
int UserCount = Convert.ToInt32(Application.Get("Count").ToString());
UserCount -=1;
Application.Set("Count",UserCount);

then go to your page and add a Label and name it lblCount

use this code to get the application visitor in the lblCount
lblCount.Text = Application.Get("UserCount").ToString();