This setting is the maximum number of authorized (displayable, post-auth-check) results to try to establish, during results authorization. Increasing this limit makes it more likely to get an exact hit count for a search (instead of a single page), at the expense of more search time and more origin server load.
The minimum (and default if empty) is the same as the Results per Page setting (here), which produces a page of results the fastest. The maximum is -1 for no limit, i.e. continue until all results are checked, or Max Docs to Auth-Check or Total Auth Timeout is reached.