When calculating coverage, only the settings "Gender", "Age", "Geography" (from the "Cities and regions" and "Local targeting"), "Advanced social features" and "Interests" are taken into account.
If you change the settings and the numbers in the forecast remain the same, then you are adding settings that cannot be used to build coverage, for example, Audience segments.
If you've targeted
segments, you won't see the number of users. The "Coverage" column will have
the status "Not yet supported". We have done this to display only the correct coverage values, and are working to display it correctly in all cases.