I won't go into detailed setups, since Blizz has done quite a wonderful job in tailoring content to suit variable and flexible grouping. What i will do is to list some recommendations tailored to each instance. For each instance i am assuming the group is doing it for the 1st time.
Gruul's Lair
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2 Prot Tanks (pally/druid/warrior) - Most pulls in Gruul involve 2 or 3 mobs that are best tanked separately. Gruul himself requires 2 tanks, 1 to MT and another to absorb the hateful strike.
2-3 Offtanks - The highly recommended setup is to have 1 OT per mob with the raid to taunt when brute charge/aggro wipe or when Gordokk priest does Psychic scream. In addition, the HKM fight requires 2 OTs for Olm the Summoner and another OT for Blindeye the Seer.
1 Mage - HKM fight requires a mage to tank Krosh Firehand.
1 Warlock - HKM fight requires a warlock to enslave the felhunter that Olm the Summoner summons
2x Shaman/Hunter/Warlock (or any combination that involves at least 1 shaman or 1 hunter) - Kiggler the Crazed has to be tanked by 2 ranged classes. The recommendation is to have 2 hunters in the same group with 1 putting up Aspect of Wild, but shamans can also put up Nature Resistance Totem and aggro from range with lightning bolts.
6x Healers - This is the bare minimum to ensure each tank for the HKM fight has a personal healer + an additional healer for MT (Maulgar hits quite hard).
Total: 16-17. The remainder will consist of dps or healers.
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Judge a man by the trials of his shield, not the empty reaping of his sword.
