Count Related Instances returns how many records in a relationship meet your conditions. Start on the object receiving the answer: count direct reports on Worker, candidates on Position, or open positions on Supervisory Organization. The related collection determines what one counted instance means.
Zero is useful. An organization with no open positions should have a count of zero, while a missing single manager relationship would produce a blank lookup. Filtering the parent report to people with some related record removes the zero cases before you can compare them. Keep the full parent population when the question asks which teams lack coverage. These exercises count stored records, so two candidates are two instances even when they applied to the same position.