An ‘Action’ in Matomo (Piwik) is counted whenever a visitor:

In Matomo reporting interface, all reports related to your visitors’ actions are listed under the left menu category “Behavior”.

To find the number of actions tracked per month, go to Matomo, and open the “All Websites” dashboard. In the calendar, select “Month”, then click on the previous month, and click “Apply”. The number of actions per month is displayed at the top next to the “All Websites dashboard” title.