I always try to put on just one thin coat/year. Bottom paint will build up to a cracked mess if it's too thick, requiring a big PITA job after a short time to remove it all. Manufacturer's recommend 2 coats for an obvious reason, kinda like lather/rinse/ REPEAT with shampoo.
There's really no point in having any kind of heavy build-up. My boat takes one gallon. Remember to save a bit in the bottom of the can to quickly fill stand pad and keel block areas once in the slings!