I checked the documentation for the Instagram API: https://www.instagram.com/developer/endpoints/
The problem is that I can't find an endpoint to retrieve the reach (how many unique people saw a specific post) nor an endpoint to retrieve the number of engaged people.
For example, on Facebook these 2 metrics are called
post_impressions_unique and post_engaged_users.
The question is if you know a way to get those metrics using the Instagram API? If not, is it going to be possible in the near future?