Manual:$wgCountTotalSearchHits

Details
Set to true to have the default search engine count total search matches to present in the Special:Search UI. This feature is currently supported only by default MySQL and SQLite search engines.

This could however be slow on larger wikis, and is pretty flaky with the current title vs content split. Recommend avoiding until that's been worked out cleanly; but this may aid in testing the search UI and API to confirm that the result count works.