1 Quart of RustSeal should be enough to coat a standard car frame with two coats. If you want to do the entire under body carriage and frame with two coats, you will want to get 2 Quarts of RustSeal.
Please be sure to read the RustSeal Directions for proper prepping of your under-body carriage and frame.
The Frame Coater Kit or the Large Frame Coater Kit are convenient kits that give you everything you need to prep and coat your chassis.