The box

Clay Jensen receives a box with seven audiotapes which were recorded by Hannah Baker.

There are 13 sides of spoken text on them.
Each side features a certain person and gives one reason why Hannah committed suicide.
So it is 13 sides, 13 people, 13 reasons.

Hannah made two sets of tapes.
One set is being passed around among those 13 people.
Hannah's rules are: 1. listen, 2. then pass it on to the next person in the story.

Hannah gave the second set of tapes to someone who is making sure that everybody follows the rules: "You are being watched." (p 17 l 16). If somebody does not, the second set will be publicly released.