For the Monastic Tradition "Way of the Four Elements" in the D&D 5e Player's Handbook (p. 80-81), some Elemental Disciplines allow the monk to spend ki points to cast a spell. For these spell disciplines, additional ki points can be spent to cast the spell at a higher level than its minimum, but there are specified limits to "the maximum number of ki points you can spend to cast a spell in this way".
But other Elemental Disciplines are not called spells and not implied in any way to be spells. Three of these Disciplines do provide a similar way of increasing the effects by spending more ki points. Fangs of the Fire Snake allows spending just 1 additional ki point for a total of 2 points.
However, both the Fist of Unbroken Air and the Water Whip disciplines say you can spend 2 ki points to cause the initial effect, "plus an extra 1d10 bludgeoning damage for each additional ki point you spend".
Is there a limit to how many ki points a Monk can spend on either Fist of Unbroken Air or Water Whip, or how much extra damage they can do, other than the Monk's total supply of ki points?
Just from reading my Player's Handbook (First Printing August 2014 "includes corrections"), it looks like the answer is no, a Monk can always spend any amount of ki points they have available for these disciplines, since the only limits mentioned are for spell casting, and these disciplines are not spells. But this seems surprisingly inconsistent, and a rare potential for doing a huge amount of damage with one action. Has anything changed or clarified this otherwise, in later revisions, errata, or comments from the designers?
Best Answer
There is no limit.
As you correctly note, there is no hard limit to the number of Ki points a Monk can spend on a single power, aside from their normal maximum number of Ki points. I checked the most recent errata, and there's no mention of limiting Monk Ki spend. If a 20th level Monk wants to spend 20 Ki points to deal 21d10 damage with a single Fist of Unbroken Air, there's nothing stopping them.
It's generally not a good idea to spend lots of Ki at once, though. Each Ki power has a much more interesting effect than just damage (like pushing and knocking prone for Unbroken Air), and will generally be more efficient at dealing damage if you spend fewer Ki points at once.
Once a Monk does their big 21d10 attack, they can't do much else until a rest. Other classes are much better at burst damage, and can generally keep doing less potent class actions once they've spent their big attack, which a Monk can't do here.