Today we’re introducing a new field called approximate_count_64bit on the Marketing API’s Saved Audience endpoint, which supports larger numbers, updating the previous bit limit on the global potential reach estimate that will replace the current approximate_count field. When querying data from this endpoint, use approximate_count_64bi instead of approximate_count, which will update the global estimate to reflect current figures. The original field will remain available for now with a 32bit limit, but will eventually be deprecated.
Potential Reach estimates how many people an ad could potentially reach depending on the targeting and ad placement options advertisers select while creating an ad. It isn't an estimate of how many people will actually see an ad, and may change with time.