I saw a sign in an elevator that stated the capacity as "15 persons". What is the rule for when to use "persons" vs. "people"?
People is always plural, so you can only use it with 2 or more persons. It is often used to refer to persons as a group in general.
Person(s) can be singular or plural, so you can use it with 1 or more persons. It is more suitable for counting as in the case of the maximum number of people allowed in an elevator.