There is no simple answer on how many DokuWiki installations exist, because that metric is not automatically tracked.

There are however a few indicators on the popularity of DokuWiki:

• Popularity data as collected by the popularity plugin.
  • this data is transferred by active wikis that have automatic submission of this data enabled
• BuiltWith
  • counts publicly available DokuWiki installations
• moz.org Backlink checker
  • shows the number of domains linking back to DokuWiki.org - not all of them are wikis themselves of course

Using these indications, it is estimated that there might be about 50 to 250 thousand installations worldwide.