Potential reach is the number of monthly active people on Facebook that match the audience you defined through your audience targeting selections.
In the API, you can query for the reach of a given targeting spec through the:
Note that although targeting is defined on the ad set,
/reachestimate is not an edge off of the ad set object.
These endpoints return the potential reach as well as bid estimates.
The bid estimate returned may vary for the same targeting when called from different ad accounts. Bid estimation takes into consideration historical ad account information and uses it to form a custom estimate.
Facebook interfaces currently return estimates for daily reach. The API only supports monthly.