Methods to test Unity WebGL multiplayer game server scalability

I am looking for a method to test the scalability of my WebGL multiplayer game (built in Unity 3D). The game is currently based on the PUN (Photon Unity Network) library and cloud service for multiplayer communication. I would like to know how to efficiently find out if the server hardware and architecture can support, let's say, 20, 50, or even 100 players in one room, with the limited number of computers at my disposal. Ideally I would like to know the frame rate each player will experience. I do have access to some powerful servers. A preliminary idea I have now is to run a bunch of virtual machines on these servers, each of which runs a browser tab with the game. Just want to know the industry practice or what you think would work. Thanks!