Yup seeing as he killed 200 rats in one attack, he gets 1 cleave attempt, not 200.

That and all the other reasons above of course. Though I would not tell him any of that until he tries it of course.

DM "You drop 200 dead rats on the floor"
Munchkin "oh I forgot, I had a bottle of air in there with them"
DM "ok you drop 200 live rats on the floor"
Munchkin "I whirlwind attack them"
DM "You don't have the prerequisites for that skill, 200 rats attack you, 10 of which get natural 20s, take 10 points of damage. Oh and while you are at it make a fort save, one of those rats was carrying a disease"
Munchkin "err I figure out some other cheesed up way that noone on the boards thought of to kill them all in one go, I now get 200 cleaves"
DM "no you don't, you killed them in 1 go, have 1 cleave"

and so on