config settings to restrict applications for 100 users only

Write code in global.asax file which interacts with the entire application.
The sample code is given below:

void Application_OnStart(Object Sender, EventArgs E)
{
       Application["CurrentUsers"] = 0;
}

void Session_OnStart(object Sender, EventArgs E)
{
       Application.Lock();
       Application["CurrentUsers"] = System.Convert.ToInt32(Application["CurrentUsers"]) + 1;
       Application.UnLock();
}
void Session_OnEnd(object Sender, EventArgs E)
{
       Application.Lock();
       Application["CurrentUsers"] = System.Convert.ToInt32(Application["CurrentUsers"]) - 1;
       Application.UnLock();
}

Hereafter you can handle the scenario as per your need.

Details here: http://www.aspdotnetfaq.com/Faq/How-to-show-number-of-online-users-visitors-for-ASP-NET-website.aspx[^]