On the 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 Piwik 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