Kara: Two for some fights, One for others.
Gruul: Two for Gruul, as many as four for Maulgar
Magtheridon: As many as Five (depending on pre or post nerf).
T4 Summary: 2-5
SSC: 2-5 (Vashj is 2, Fathomlord is 4).
TK: 1-4 (Solarian is one, Kael/Alar is 4).
T5 Summary: 1-4
T6 Summary: 1-3
Trend: The number of full tanks needed shrinks as you go up in Tiers (and I'm going with the assumption that you want a full time tank for progression fights) . On top of this, you assume that your going to recruit over the minimum, so that there are backups. Then there are considerations of tank types: you most certainly want a protection paladin for Hyjal, but you can't (without significant effort) have them tank Archimonde. Add a backup to each, and your looking at four tanks. Add another Protection warrior as a backup for Kael (where you may want two warriors, in addition to a protection paladin and a feral druid) - and now you have potential for five full time tanks logged on for a one tank fight (ie, Solarian, Archimonde, Najentus, Teron, RoS).
I'm curious to see how many tanks new 25 Man Raids will want. I'm excited that warriors and paladins will be generally interchangable, but concerned that it will put a squeeze on LOKI's fairly large tanking base (Three Protection Warriors, One Protection Paladins ((with a Holy that wants to Tank in Wrath)), and two Feral Druids). Worse off, I did the recruiting all in good faith, but now am not leading the tanking scene - leaving our new Tank Officer to deal with sorting it all out.
All of our tanks are great, and I don't want to leave any out - which is why I'm hoping for some generally higher number-of-tank requirements in Wrath. Hopefully, with the addition of the psudo-tanking Death Knight class (none of which we have players indicating strong desire to switch to) - encounters will be designed to better fit more tanks.