There is no such rule about how many people are allowed in support chat, but I do agree that 3 is more then enough. Otherwise to me it feels like to much of a crowd. That's what I think anyway.
Also the people that come to support for well support, I know I just leave if there's to much of a crowd or people are being unsuportive. So yes I do agree.. if there are already a few people in support that is enough. Also please if you need support yourself you can always make your own room...