A spellcaster can cast any spells into the ring, so long as the total spell levels do not add up to more than 5.
Apparently it can hold infinite number of 0 level spells?
Huh, apparently.
Other stuff that deals with spell levels (like the magic item pricing guidelines or the Mnemonic Enhancer spell), usually come with a caveat that level 0 spells count as half a level, which I imagine most GM's would tack onto this case via houserule as well.
But 50,000 gp and a ring slot for being able to cast all of your class' cantrips might not be an unfair investment. I rather like the idea.
As written, yes. This is a bug due to copying this item from earlier editions of the game which didn't have 0-level spells, and not thinking it through properly - and it's not the only one!
It's actually not such a big deal in Pathfinder, as cantrips can be used over and over again by many casters, anyway.
