In Office 365 there is an option to enable an add-in for spam reporting. This tool can be used to measure how many users in your organization that reports fraud emails when running a simulation using Fraud Risk Assessment.
It's recommended to set up a new email account for the copy of all the reports from your organization. By doing this you can count the actual number of reports received from your users.
You can read the guide how to set this up here (external link):
When the add-in has been enabled you can then access the report BCC email to see how many users have reported the phishing email through the assessment.