How are weekly potential impressions calculated?

Weekly potential impressions are available on the ad set level, except for Broad Targeting, Website Traffic Audience Targeting, and List Match Audience Targeting. The latter two have their projected volumes in the Audiences tab. 

When your Primary and Secondary targeting is selected, an estimate is made for the max impressions your ad set could receive if it wins every auction it participates in. Forecasts are based on a sampling of the prior week's inventory. The number range is meant to provide you a very rough estimate of potential inventory and may not represent what your ad set actually generates. In some rare cases, the estimated impression range can be reduced when you expand your targeting criteria due to the sampling method.

In general, if the estimated potential impression range is low relative to what you want to achieve, we recommend expanding your targeting criteria.