Jump once? You'll get Jump Boost 1!
Jump one hundred times? You'll get Jump Boost 100!
Drinking milk will decrease your Jump Count by 10, but you can only have one milk bucket in your inventory at a time, and you can only drink milk once your Jump Count is 15 or higher, so you can only get back down to a minimum of Jump Boost 5.
You can reset everyone's jump count using:
/function increasing_jump_boost_43615:reset