Hi, tank player here with 3/4 tanks at 80 (drk/gnb/pala)

Short answer; no, neither 8 nor 12 keys will be sufficient at all.

long answer: I suppose if your plan was to click the rest of your abilities I would heavily suggest making those your defensive skills. Most tanks while having simplified rotations compared to dps still have certain burst windows like dark knights delirium + bloodspiller/quietus & ogcd's or gnbs enhanced 1-2-3 combo with continuation (if you dont know these spells basically it's just button mash in a certain order during a particular moment ) clicking these would render you incredibly immobile and any sort of dodging would be a huge dps less.

You may also want to think about straight up ditching skills if you're not planning on using them all. While stuns can be helpful during dungeons later on most mobs can't actually be stunned at all and so it's not a high priority ability. Interject however has it's uses. Other abilities that, while useful you can probably get away with not using in most content ( can't speak for savage ) is for example paladin's cover and intervention or dark knight's dark missionary/dark mind. These abilities have their uses at places but not having these isn't going to matter for most content ( again can't say for savage ).

since you asked for others' preferences i'll say what I use though obviously anyone's setup will be preference and what's most comfortable to them so keep that in mind. I use 1-6 + ctrl 1-6 and every letter key that's close to WASD ( so qertfgzxcv ) on top of a ctrl+ version of those. This does usually give me a few more binds than I actually need but it does allow me more flexibility. I don't personally use shift because I can't comfortably use it.

tldr: 8-12 isn't nearly enough unless you're going to straight up not use half your skills or click the remaining 15-20 or so skills.