It depends on what kind of virtual SA are you referring to.... if its a DTE ( Demo edition) it supports only 3 concurrent users and there is no license available to expand it , its more like a demo test box that can run on VMware. If you are looking for SPE ( Service Provider Edition) its a lot different and as far i know the minumum license starts with 2000 simultenous users  but the actual thing lies in the hardware that is being used to deploy these if you have resources on your VM that can handle 1000 concurrent users go for it. But from my experience i have seen performance degradation on one of Sa-4500 i have , juniper says its suports 1000 concurrent but really after 600 users the performance drops significantly . You can find more info here http://www.juniper.net/assets/us/en/local/pdf/datasheets/1000320-en.pdf ... View more

i think it depends on your cluster type. if you are using an active/active, best to do 100/device to make sure you can host all users on either side; if you are using active/passive, 1x100 should be fine your SE can help you our=t best when ordering as well ... View more