that won't work. It will count the number of requests in a single session
You'll need a static int in a servlet that is guaranteed to be called by each session, plus an object in the session itself that is used as a trigger (if it's there don't increase the int, if it isn't there increase the int and put the object in the session).
Make sure you decrease the int again when you invalidate the session.
This is NOT guaranteed to give the actual number of sessions of course.
Most sessions will never be properly invalidated therefore the counter will not be decreased for them.
There is no way around this.