You can see how many customers have checked in to an event by using the Reports widgets.
1. Log in to the admin page with the URL extension /admin/plugs
2. Click on Reports on the lefthand side menu
3. Click on "Checkins" in the drop down
4. Select the desired event
5. Set the Date Range to All Time
6. Click Filter
*if you'd like, you can also print out the results with the "Print" button