In Office 365 there is an option enable an add-in to report spam, this tool can be used to measure how many in your organization that reported the phishing email recieved through the 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 number of reports recieved 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.