Thread:Project:Support desk/Howto add number of included categories, pages, and files to subcategory listings?

I've set up an intranet wiki (Mediawiki 1.21.1, PHP 5.4.9-4ubuntu2.2 (apache2handler), MySQL 5.5.32-0ubuntu0.13.04.1) and wonder why no subcategory statistics show up when browsing the category tree (i.e. there is no hint "The number of included categories (C), pages (P) and files (F) is stated in brackets." and also not the numbers in brackets). I can't find the feature among the applicable settings. On the other hand, this feature seems to be active on all "real live" wikis I know and I've gone through the lists of extensions installed at some of these wikis – without noticing that any of them might be responsible.

So I wonder: What am I missing to activate this feature?