Manual:$wgWatchersMaxAge/en

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 approxymate information is shown.