By default, the Owner of the Event is the Sales Rep. who created it, and can be different from the Organizer.
In order to create a Search which calculates the number of Events created per Sales Rep., the user needs to set up a Search as following:
1/ Navigate to List > Search > Saved Searches > New
2/ Select Event as the search type
3/ In Criteria tab > Standard subtab > add "System Notes Field: Type=is Create"
4/ In Results tab > Columns subtab > add the Field "System Notes Field: Set by", then use the Summary Type Group.
5/ Use the Summary Type Count for the Field "Event".
The Search Results can be narrowed further by using additional criteria, such as a time period, specific attendees, etc.