Reach is the total number of unique profiles that have seen your Instagram post or story. For individual posts, this information can be found on the “View Insights” button under the post.
When it comes to overall account Insights the Reach gives you the total number of unique profiles that have seen any of your posts, in the given period of time. This information can be found under Settings > Insights.
Impressions statistic of your posts is the total number of times that your post was seen by users. This includes multiple views of the same user.
For example, let’s say that a single account views your photo 5 times while scrolling the news feed. This will give you +1 Reach and +5 Impressions.