It really depends on what you're going to use the truck for and what the manufacturer suggests. Generally, you would apply 2 coats. I've heard show trucks that had 4 coats applied. It depends how deep you want the paint to look as well.
You can shoot a hundred coats if you want... the main thing is if you shoot another coat too soon, it'll run or sag. Too thick and the same thing, too thin and it'll orange peel. Wait too long and it may not adhere correctly. There is a very fine balance.
So far on mine I found that two "wet" coats is pretty safe. Three is fairly safe on horizontal surfaces, probably will sag on verticals. Four...forget it. I used the TCP Global KC2020 urethane clear. Really will depend on the product you use. Look at the info sheets that come with your clear coat and shoot it based on that.
There is a big difference in the quality between clearcoats. You absolutely get what you pay for. Cheap clears are low in solids and do not last long. . 2 coats of cheap clear will not give you the same dry film build as a quality high solids clear. The uv light gets to them first. You may only get 3-4 years before they peel. In contrast a quality clear will last 20 years or so. There is a balance of quality and price. you can spend $100 a gallon or $7-800 for clear. I have shot many, many gallons of Matrix MS-42 clear with great success. I have a vehicle that was painted with waterborne base and MS-42 back in 09' that still looks great. It has sat outside most of that time . I shamefully haven't waxed or polished it in many years.
The slower clear dries, the more gloss it will have. Use a fast hardener and it will "dye back" or "loose some of it's gloss" in a few months. Its easy to spot the production, collision shop's repair work. With a high solid clear, you do not need spray more than 2 coats usually. If you do spray more than 2 in one go, you will get "solvent pop." This is where the top layer dries and seals solvent in the lower layers. It will come out causing blisters in the clear. The only fix is to sand it back down and respray. The more clear you spray, the more texture will be in the finish. If there are too many layers of paint it is more likely to crack at some point. This includes the original paint and collision repair work done in the past. For a show car finish, spray 2 coats and the next day, block the car down smooth with 1000 grit and wait a couple of days for it to cure out. Longer in cold temps. Respray 2 more coats of clear. The following day block sand and polish. You want to do this within a week when it is tender. Wait 6 months and you'll work yourself to death. After a couple of months, polish it again. The clear will loose some shine and some sanding scratches will come back.
MS-42 clear to me is more difficult to shoot than other clears, but you learn it and it's no problem. It has some quirks. Do not spray when the panel temp is below 65. It will fish eye and run off the panel. Been there, done that. Don't spray it completely how you want it to look. It has a long post flow, almost like polyurethane paints... Imron. It is easy to run and sag. But it's a great clear and reasonably priced for what you get.