2 good heavy coats of urethane primer was between 3 and 4 quarts on my '73, so if you are figuring a gallon you should be fine. If your car is bare, then 3 coats. I stopped using sealers, the modern 2 stage urethane primers are nearly bullet proof. On color coats it is about the same amount, but with base/clear you will need a gallon of each, of course. Base mix goes pretty far, but I always figure on a gallon to make sure that I have leftovers for future repairs. Today's clears are so user friendly compared to 20 years ago, you should do fine, and I usually shoot 3 medium wet coats with a tad extra reducer, to flow it out. The reason is to avoid runs, if you bail it on there is always that possibility, and they are only easy to fix when they are in a good spot, which is usually NEVER!! My runs always seem to end up on body lines, by marker light openings, lower rear quarters, or anywhere that the razor blade or sanding method of repair is USELESS!