What is reach?
Reach
The number of unique accounts that saw a post at least once, as opposed to the number of times it was displayed.
Reach counts people; impressions count views. One person seeing a post four times is one reach and four impressions, which is why the two numbers diverge sharply on content that gets revisited.
Reach is only available to the account owner through Instagram's own tools. Any third party quoting your reach is estimating it, usually from engagement, and the estimate degrades badly for accounts with large inactive followings.