You can add a column showing how many result notifications each registrant in your Race has registered for to your Participant Report by going to Participants > View Participants > Report Options > Hidden Fields and checking this box:
Now that you've enabled this field, it will be available to you when customizing this report under ReportOptions > Customize View > Pick from All Columns > No. Result Notifications Setup. Once you have this checked, be sure to click the Customize button to save the
changes to your report.
In your customized report you'll now be able to view this data: