Because there are so many differences between machines with respect to CPU, memory, etc. I've had some success with running the "Evaluate Agent VUser Capacity" built into Silk Performer. Open a project you want to work with, and within the PROJECT pane expand your Agents - right click on the agent you want to check and select EVALUATE AGENT CAPACITY - this should give you a rough idea. Given Silk Performer runs as x32 - notwithstanding we run it here on x64 windows systems, we get anywhere from 25 to 35 virtual users per agent. We ended up setting up 20 virtual machines (made one and cloned it 19 times) and this seems to serve us well - however we only have a 500 VU license, so when you get much above that I imagine it might get a bit dicey. I used to push the limits and go up to 45 - 50 VU's per agent, but got many, many time outs and other weirdness that was hard to explain, to this is probably one area one needs to be conservative with.
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