How often has been Mediawiki downloaded until now. Is there somewhere a page with information about that. I currently want to get a overview about how much software that is partly developed by volunteers is used.
Topic on Talk:Download
I don't know if download counts would give you meaningful information about software usage. Note some hosting companies offer one-click installs and other solutions offer bundled software. Also, it may be available from linux distros.
WikiApiary has stats of live wikis using MediaWiki. https://wikistats.wmcloud.org/ may also be useful to get numbers on wikis inside wikifarms.