Manual:$wgContentNamespaces

Details
$wgContentNamespaces exists to allow custom namespaces to be acknowledged as containing useful content for the purpose of Special:Random, Special:Statistics, and. Useful if additional namespaces also contain "content" which should be considered when choosing a random page or generating a count of the number of articles in the wiki.

The main namespace is considered a content namespace, as are other namespaces in this array, referenced by index.

Example:

$wgContentNamespaces = array (0, 12, 112);