You can add any kind of statistics service to your dokuwiki.
Let's start with the more general way:

• go in libs/tpl/dokuwiki/
• create the file footer.html
• insert here the html code snippet for the stat service you use

That will “survive” any dokuwiki upgrade because it is treated as a user document.

If you want to add Google analytics statistics, you can follow the above or simply install this plugin https://www.dokuwiki.org/plugin:googleanalytics

Once installed through the extension manager, you have to go into the configuration manager→Googleanalytics and add your Google Analitycs ID (the alfanumeric code you see in the dashboard of you Google Analytics account).