"That creature must make a
Strength saving throw. On a failed save, the creature
takes 3d10 bludgeoning damage, plus an extra 1d10
bludgeoning damage for each additional ki point you
spend, and you can push the creature up to 20 feet away
from you and knock it prone. On a successful save, the
creature takes half as much damage, and you don't push
it or knock it prone."
The way I see it you have to declare your ki usage when you use the ability, same as when you choose the slot level for your fireball or whatever spell you use. The text is written a little differently due to how the monks abilities work. I wouldn't count as a spell probably, meaning it would have no limit, but that isn't that impresive. 21d10 once per short rest vs Save and take 10d10 or die 6 times per short rest for the open hand monk.