How do I access a real time counter of visitors currently on my site?

On the Matomo (Piwik) dashboard you may add the widget « Real time > Real time visitor count ». This widget shows a counter of the number of people currently on the website.
By default Matomo uses a 3 minutes delay to define a « Visitor currently on the website ». You may edit this value in the config.ini.php file, for example set to 10 minutes:

[General]
live_widget_visitor_count_last_minutes=10