A tank can only survive with 2 mobs on them. If you have 3 (or is it 4?) you get 1 shot (and a red text telling you).
If you are tanking 2 mobs on a tank, you will want to make sure you have a lot of health (10m+) and good priests. Because the max hit with 2 on you is just under 5m. So if both of them hit you at once, you need to be able to survive it. And for some reason, my tank who tanks 2, also ends up tanking the guardian, so that's another mob tossing around some hits.
Looking at ACT, my tank who tanks 2 mobs, has a income damage of 2.7m DPS.
Tank who tanks 1 mob has 1.3m DPS.
And lastly, the shitty 3rd tank 1.65m DPS.
If you want to go 4 tanks, you should have no issue with tank damage. If you want to maintain aggro well, you could use 2 tanks each tanking 2, but the pull is really annoying, because if 3 hit someone, they are likely dying, and if they die it's a wipe.
I find 3 tanks makes it a cake walk as long as the 1 tank who tanks 2, is beefy enough.
From the priest side...
My defiler did 3.4m HPS and my warden did 700k HPS (on the Guardian tanking 2).