Parry hasting happens in this fight. Saw a tank drop like a !@#$ing stone last week when our melee got careless... Two parries and a Sindra melee hit flattened our tank in a flash.
As far as the number of healers... we run 8 for this fight. Remember that at any given time, 3 of your heals have unchained, and can't cast. Leaving you with 5 to actually heal the raid + tanks.
Hope that helps.
"Faith is the assured expectation of things hoped for, the evident demonstration of realities though not beheld."