Restricting Concurrent users on a particular BSP Application

We are having an appraisals link in the PM section of the portal which runs the HAP_DOCUMENT BSP application. We have to restrict the number of concurrent users accesing this application at the same time to about 600 so that the server will not crash...Is there a way by which i can find out the no of users accesing a partcular BSP application concurrently??