13 on a pretty average quad-core Phenom with a Fusion-IO SSD - http://www.tgdaily.com/content/view/39600/135/

Granted I only got to the load screen because we didn't have enough accounts to sign in.

I could probably get 15-20 on my Skulltrail IF I had a really fast SSD and more system memory. I'm sure there are people out there who are probably doing the same thing, but have chosen to keep quiet about it. As far as games go, WoW is pretty lenient on system usage ... we're not talking about playing Crysis here.