Todd,
The amount of paint needed will depend on several variables. I don't know a lot about 1928 Coaches, but from what I have seen, many are painted with the main color on the body, with fenders, running boards, and sometime roofs in black (or contrasting color?). Obviously that would cut down the quantity of the body color. If you are painting the undersides of your fenders, running boards, hood, trunk lid, etc., the same color, that will also add to the amount of paint needed. Also, I'm not sure if you want to do a single stage acrylic enamel or acrylic urethane, or a 2-stage base-coat clear coat. In any case you will want at least 2 or 3 coats of each color to be applied on the car.
If you're using 2 colors, I would estimate that you will need at least 2-3 quarts of each color; if doing one color, certainly more. My 1938 panel truck (which has MUCH MORE surface area than your 28 Coach) required well more than a gallon of the body color. In any case, it would be best to order as much or a bit more than you need--even batches that are mixed according to a specific formula can show slight differences in tint and shade if they are mixed on different occasions.
If you are having it done by a professional painter, he will certainly be able to estimate the amount of paint needed when he sees your car in person. As you undoubtedly know already, good paint is very expensive, so you want to be sure you have a good game plan, then order your paint accordingly so that you don't have problems down the line, having to re-order, etc.
One final note about TCP Global and the Autocolor Library. As I said, I've had great success using their services and paint, but they do require a week to 10 days to mix your paint and then send it out--certainly not as convenient as going down to a local NAPA or similar and having a batch mixed up on the spot. So plan accordingly.
Good luck!
Jim