I think, it will be less than a gallon of "finish", given the reducers and other additives one needs to add. However, are you including primer, sealer? Is it bare metal or "scuff 'n coat"? Door jambs, trunk, interior etc.? What is the coverage? I'm not an authority, but, have shot all of the above.
Need more details for better answers.....
I am thinking single stage paint. the car will have already been primed and had sealer on top. this will be a total car paint. jambs, trunk exterior, not the interior as that will not be body color. So will one gallon cover? And how many coats should I use?
Ok, a gallon should do well in coverage. How many coats? As many over-lapping sweeps as it takes for coverage. It's not like one will keep shooting product. Best Practice, follow the mfgs. coverage and instructions.