<cr>
2 gallons would do your pickup. If you wanted to try buying less (like 1 1/2 gallons), then break the job down into sections (exterior, interior bed, interior cab, and engine compartment). That way if you run out of paint, the color matching isn't as critical.

Whatever you decide, remember to mix your separate containers of paint together before you start painting. This insures an even color mixture between the separate containers.

Just for your information, for a gallon, the 4th quart of paint is dirt cheap in comparison with the first 3 quarts.