You would almost have enough to do the 10 man raids, which in the next expansion will be all of them. You'd just need to fill 2 spots, or add 2 of your own. But I'd still do 5 at first. It'd be hell to try to level 8 toons at once in WoW. You can't quest, and you get extremely lower xp than normal. If I were to eventually go to 8+ accounts, I'd just level 2 groups of 5 (try a 5x shaman and a holy trinity group) separately, and if you wish to do all 8, get more accounts and xfer the chars off.

Man, Imagine seeing 8 shaman coming at you.....