Determining # of direct reports at time of termination

Summary

How can we determine how many direct reports someone had as of their term date?

Content

We have received a request to pull together a report of terminations over the last year and to include how many people the person managed at the time of termination.  I'm not sure how to pull this from the database, given how the line manager relationships are stored.  

Anyone have any advice/experience for pulling total direct reports (employee only) as of manager term date (not effective date)?