If you want to see how many people received a certain number of emails, you'd do this with a smart list.
Your filter would be Was Sent Email: Email is any, Date of Activity (I'd x it out if over lifetime or you could customize), Min Number of Times (constraint). You can make different smart lists with different minimums to see about where you fall in email volume. This will also help you determine what communication limits you may want to set.
Another filter you may want to add is email address not contains (the domain of your company).
Hope this helps!