There are several factors that enter into the amount of paint you will use. Better paints tend to cover better than lesser quality paints. The color of primer used can factor in on things. A light colored topcoat will cover over a light colored primer better than over a dark colored primer and vice versa.
If you have problems with dirt, fisheyes or with wet sanding, it can add to the amount necessary to complete the job. Also, when you mention a body tub, do you mean the outer surface only or every surface inside and out? And the type of gun you use will also enter into the equation.
You could be looking at anywhere from a few quarts to a few gallons.