If you are talking about counting how many users are using a particular application - you would need to keep count yourself. Therefore it would only work for a Servlet sort of situation where the application is running long term and users "sign on" to it. However - this question has nothing to do with how the JavaRanch website is maintained - so it belongs in a different forum. Perhaps "Servlets and JSPs".
"JavaRanch, where the deer and the Certified play" - David O'Meara