Limit theme: "Transformation limited to 5 concurrent users"
 

Does anyone have any content along the lines of a desirable transformation that can only be bestowed on a limited number of people?

For example, a spellbook makes old people young, but only has enough power to enchant 5 people at any given time. Persons #1, #2, #3, #4 and #5 read the spell and everything is hunky dory. But when person #6 reads the spell, person #1 immediately loses her enchantment, and so on.

Or a limited number of artifacts--rings, perhaps--that transform you when worn, so there becomes a power struggle over who gets to stay transformed.

The transformation can be limited to dozens of people, or just a single person.