For 10 man, 3 healers is what we have found optimal if your running without frost resist gear. A pally of some sort is almost necessary as the frost resist aura is vital. Have a single target healer on the MT, and the other two healers worry about raid heals. Replenishment is pretty much a must at the moment as many raids arent geared to down him fast enough for your healers mana pools.
For 25 man, we have been able to do it fine with seven healers (mainly multi-target healers) and everyone with at least 80 Frost resist before buffs (so basically one piece of gear). We also require flasks/food to make the fight easier. A decently geared 25 man group should have no problem with only minimal frost resist and some good healers.
I'm from Onyxia, where trade chat makes you wish you were in the barrens.