Why would it take a bunch of them to do it?
Because a ship is really big. And while Yoda can lift a fighter from a swamp, moving one against it's flight path requires enormous energy and concentration.
I am astonished at how you remembered that scene, but apparently forgot the part where he said "Size matters not." That was kind of the whole point of the scene.
He said "Size matters not" because Luke didn't think Yoda was powerful because Yoda was small. The fact that Luke couldn't lift the ship implies that the size of objects being lifted does matter.
He says more than just that:
“Size matters not. Look at me. Judge me by my size, do you? Hmm? Hmm. And well you should not. For my ally is the Force, and a powerful ally it is. Life creates it, makes it grow. Its energy surrounds us and binds us. Luminous beings are we, not this crude matter. You must feel the Force around you; here, between you, me, the tree, the rock, everywhere, yes. Even between the land and the ship.”
Yoda makes the size comment specifically to answer Luke's statement, "It's too big." To me this implies more than just Luke mistaking size with power, but also mistaking that the size of that the fighter is what is important. What is truly important is the connection between things. Luke did not fail to move the fighter because it was big, but rather because he could not get away from his own judgements and so could not perceive and affect the connection between him and the fighter.
I imagine Yoda's 200 gigatons of midi-chlorians (or whatever those stupid fucking things are called) helped too. Thanks George Lucas for ruining -everything-.