In those situations there are usually 2 or more tanks, so you should have a main tank on the Hunter's mark mob, and the off-tank grabbing whatever. Also, there should be saps, sheeps, sleeps, shackles, and seduces going on to crowd control. But when the situation absolutely calls for tanking multiple things at once, I just focus on one main mob and Demoralizing Shout at the very beginning to try and get everything on me. I sunder the crap out of one mob so I have guaranteed aggro from it, then wait for Revenge to pop up. When it pops up, I immediately taunt another mob, Revenge it, sunder it a couple times, and ping-pong attacks between the two mobs. Repeat until everything is down.
It's not your fault if things seem too crazy to handle, because usually there really are too many things to tank at once. However, just try and keep everything off the healers, and ignore any adds on rogues or hunters. Mages and warlocks should be trying to keep things off of themselves with their abilities, but if things get bad, help them too.