Hi @katu,
Welcome to the forum!
Option 1: have a separate collection where you store the progress for each story/user. Collection has relationships to Users M:1 (a progress record belongs to one user) and to Stories M:1 (a progress record belongs to one story). You create a record when a user starts or finishes the story. You can also store additional information in the record (e.g. start time, finish time, comments, etc.).
A very similar logic is described in this video (https://youtu.be/_HS9WkDJbRk) - the idea of a separate collection is the same.
Option 2: create M:M relationship between Users and Stories, and add this relationship when User finishes the Story. Here you will be able to count nb of Stories for each user and have the progress as a result.
Best,
Victor.