[haste]

It depends a lot on the boss and class. For example Skeram in AQ40 can be beaten with three people, but it's quite the RNG fight then. Doing Twins in AQ40 with three people is ludicrous tho'. Hakkar in ZG can be taken with two, but it's also the same RNG mess that Skeram is. With three people it becomes a cakewalk tho'.

ZG: 2 players up till Hakkar.
MC: 2 players should be more than enough, as most of the content can be solo'd
BWL: 3 players should be enough up till Chromaggus. Do take this with a handful of salt however... I was drunk out of my mind when I created a group for it.
AQ20: Should be as easy as ZG. We plowed through most of it with priest/pala. We had a DK joining later on due to the achievement of completing it tho'.
AQ40: Skeram is quite a PITA with few people due to his MC-ing. After Skeram it's a cakewalk till Twins tho'. C'Thun is also a joke at L80.