Considering that the wording is almost exactly the same as a ring of arcane mastery (20,000 gp) that stores up to 4 points to augment a magus' arcane pool, it looks like 2 ki points is the limit; which for 10,000 gp looks about right (half the points for half the price). The "up to" wording just means that the wearer can "charge/recharge" the ring a point at a time if desired.
The only real ambiguity is in the second additional benefit: "As long as there are at least 2 ki points stored in the ring, the wearer reduces the number of ki points needed to use a ninja trick or ki ability by 1 (minimum 1 ki point)." The wording implies that it's possible to have more than 2 points stored in the ring, but there is nothing that explicitly allows it; having 2 points as the maximum still meets the "at least" criteria.
Wait, why is the cost 10,000 gp but the creation cost 3420 gp? It should be half.
Ring looks pretty weak, I'm guessing it was meant to cost 6840 and got jacked to 10,000 gp at some point. Both because of the power of the ring and because 3420 is a much odder number to pop up in the final write up than 10,000 is.
The ring isn't weak, ki is weak. for a resource as scarce you'd expect a great deal more of it.
Well yeah. That is the base problem. But I don't see them ever fixing ki, so any ring using it and the same price structure as arcana is going to look weak.
Also, magic items generally go up in cost exponentially. A +4 str item doesn't cost 2x what a +2 str item costs for example. So I dispute that "half of what a 4 point limit item costs" is actually a fair price.
The biggest benefit (IMO) is the ki point cost reduction (see the sentence I quoted above), which is always in effect as long as there are 2 ki points stored in the ring. Because ki points are a replenishable resource (especially as a hungry ghost monk or qinggong monk with ki leech), this lets you use those more expensive ki powers much more often.
As an example, a 12th level version of a Skull and Shackles character concept:
19 Str (+2 race +1 advancement +2 belt), 16 Dex (+2 tunic), 13 Con (+1 advancement), 10 Int, 18 Wis (+1 advancement +2 headband), 7 Cha; 15-point buy
Favored class: monk (+1/4 to ki pool)
12th-level qinggong monk ki power (first level to choose an alternate power): dragon's breath (2 ki points, 1 point with filled ring of ki mastery)
Skills: Acrobatics 8, Climb 3, Knowledge (Local) 8, Perception 12, Profession (Sailor) 5, Sense Motive 12, Stealth 12
Feats: Combat Style Master, Dragon Ferocity, Dragon Style, Elemental Fist, Improved Grapple, Improved Unarmed Strike, Marid Style, Punishing Kick, Snake Sidewind, Snake Style, Snapping Turtle Clutch, Snapping Turtle Style, Stunning Fist, Toughness
Gear: amulet of natural armor +2, jingasa of the fortunate soldier, snakeskin tunic, vambraces of defense, +1 light crossbow, bodywrap of mighty strikes +1 cruel, +1 ki focus temple sword, acrobat slippers, anaconda's coils, cloak of the manta ray, handy haversack, headband of unshakeable resolve, ring of force fangs, ring of ki mastery, 4,035 gp in consumables and non-magical equipment
I agree. It's not the cost that bothers me, it's the fact that ki is weak.
Agred as well. Also, since it's a difficult source to replenish, though rage rounds seem to be just as hard to get back for rage characters, the ki powers either seem worth it sometimes or not worth it most of the time if you can't get back ki somehow. Ring of ki mastery is a great way to stretch out many of the 2+ ki point powers, such as abundant step.