Morchok requires 2 tanks on both Normal and Heroic.
Yor'sahj only requires 1 tank on both Normal and Heroic.
Zon'ozz only requires 1 tank on Normal, no data on Heroic yet but it's safe to assume the worst.
Hagara requires only 1 tank on both Normal and Heroic.
Ultraxion requires 2 tanks on both Normal and Heroic.
The gunship requires 2 tanks on both Normal and Heroic.
Spine of Deathwing requires only 1 tank on Normal, not sure about Heroic but we can assume the worst again.
Madness of Deathwing requires only 1 tank on Normal, yet again we can assume the worst for Heroic.