The Reach Rate for a given Story is the number of people who have seen the Story divided by your number of followers at the time of the post.
For example: my Story has reached 2 000 persons and I had 20 000 followers at the time of the Story. My Reach Rate is 10%.
The Reach itself refers to the number of different people who see your Story. One person can see your post 5 times but is only counted once toward Reach.
For example: If 100 different people see your Story twice, your reach on this Story is still 100.