Tanks
3-4 (Paladin tanks for first two bosses, Warrior for Archimonde)
Healers
6-9 (Higher count for Anetheron and Azgalor)
DPS
12-16 (Hunters and Mages highly valuable for AoE and utility)
Paladin tanks are recommended for wave clearing. Warriors with Berserker Rage are essential for Archimonde's Fear mechanic.