Manual:$wgArticleCountMethod

Details
Method used to determine if a page in a content namespace should be counted as a valid article.

Redirect pages will never be counted as valid articles.

This variable can have the following values:
 * 'any': all pages as considered as valid articles
 * 'comma': the page must contain a comma to be considered valid
 * 'link': the page must contain a wiki link to be considered valid
 * null: the value will be set at run time depending on $wgUseCommaCount: if $wgUseCommaCount is false, it will be 'link', if it is true it will be 'comma'

Retroactively changing this variable will not affect the existing count, to update it, you will need to run the maintenance/updateArticleCount.php script.