This would be tough with the editor since when it simulates clients, it run multiple instance of the game on the machine. This makes the machine become the bottle neck instead of the server. To get a good indication. I would run the server on one machine then right a script that slams the server with thousands of requests and see how fast the server handles a request for a given load.
I would recommend reading the below, this is where I learned queueing theory and stress testing:
http://cs360.byu.edu/static/lectures/winter-2014/queueing-theory.pdf
http://cs360.byu.edu/static/lectures/winter-2014/performance-measurement.pdf