Unless I have missed something in the responses (it wouldn't be the first), the main point is missed. "Coats" of paint is too broad. It is like saying a bunch of paint. Coats don't mean anything. What matters is mils (sp.?) of paint. A "normal" coat is about 1 mil which is the thickness of the clear plastic around a pack of cigarettes. A modern urethane base/clear paint job consists usually with three coats of base and three coats of clear. This usually equals around four mils of paint, not six. The base is not usually as thick as the clear. I say "usually" because it all depends on the painter, brand and mixture of paint, paint gun, paint gun settings, air pressure settings, and personal technique, to determine how many mils thick a "coat" of paint is. I might put 3 layers of silver base, mist 3 layers of silver effect pearl, then mist three layers of blue pearl on top of that before I even get to the clear. This is six coats already, but probably only 2.5 mils of paint, someone else might use 2 coats of base that was 3 mils thick.
On how much is too much, assuming you are starting with a freshly stripped and primed piece of metal. Three mils of clear is fine for most jobs. If you are going to wet sand and buff the finish, then 4 mils. You will end up with 3 mils if you take it easy and don't have too much orange peel to start with. I have put 10 mils on for customs, but you MUST follow special procedures.