The biggest thing that can change how much color you need is simply, are all the things being painted the same substrate color? Is that color similar to the color being painted so it will cover easily?
Say you have the car in gray primer, and there are black primer spots all over, that is a disaster waiting to happen as it will take many more coats. If you are using a value line cheaper paint, add many more coats. So if the whole thing were gray and the paint going over it is light green, you are going to use a lot less than if some parts are gray and others are some other color.
If you use a value line paint you are going to use much more, so think about what paint you plan on using and maybe the top of the line isn't that much more when you factor in how much you will use.
All I know is what I have lived by, buy PLENTY! The paint is damn cheap when you think about running out in the middle of the paint job!
Having a quart or two left over to put away for some future mishap or to paint that new cowl induction hood you decide on getting......