It all depends on max level, f2p, or not.
3 groups minimum IMHO. Three tanks (Your choice but WAR are kings for tanking raids), 4 Heals(2 x CLR minimum+1 SHM + Any), 3x BRD, 1 ENC, rest DPS your choice. If F2P and no augs, stick with casters even though currently melee is king in the...