If its true that wow does not write to drives untill log out, then with a SSD drive I don't really see 10 or 15 as an upper limit.

Rather then the resolution of each client the total screen resolution is what you need to minimize as that is what you have to render. (i.e. you put 10 800X600 on one screen isnt less then one 1024 X 786), thats why I prefer the full screen each client type set up.

You really won't know unless you have a SSD drive though.