One quart should do the job. Opaque single stage colors will get full coverage in less coats than a base/clear job. Make sure that if it is going red/yellow/white that the primer is a light tint (white/yellow/light grey ect.) as it will use less paint. Trying to spray red on an upper that is white and a tank that is currently black will lead to the tank needing 3 to 4 coats (instead of 2) and it may still seem an off shade.
BTW......O'Gradys Paint FTW!!!! (my paint jobs are done with a special paint made with a space age molecular make up that allows the air to pass with less resistance, in other words my paint jobs are the fastest....scientifically tested in my lab/garage and on the race track, lol)