If you optimize everything right you should be able to get away with 5. Vista is far from optimized compared to XP but if you have 64bit Vista and 4GB of ram with the rest of your specs you'll probably be OK.

I'm running XP32, 3GB of RAM, 2.2GHz AMD X2 overclocked to 2.53GHz, 512MB 9800GT and I am running 5 copies of WoW with the main window at 1280x960, alts at 800x600, 2x FSAA on the main window 45fps or so. One of the main keys is to symlink your data/cache directories from your clones to the main, that helps a ton.