Manual:$wgWatchersMaxAge

Details
Counts the number of active watchers of the page that are displayed in and the  field of the  API if  is enabled. Per "active watcher" it counts watchers that have visited that page since the last edit or that have visited the page at least  before last edit (as tracked by the field).

If the viewer does not have the user right and either the number of active watchers is not above  or the number of total watchers is not above , only approximate information is shown.