One Level At A Time

employees:

WITH RECURSIVE chain(id, depth) AS (
    SELECT id, 0
    FROM employees
    WHERE manager_id IS NULL
  UNION ALL
    SELECT e.id, c.depth + 1
    FROM employees e
    JOIN chain c ON e.manager_id = c.id
)
SELECT depth, count(*) FROM chain GROUP BY depth ORDER BY depth;

What is the output?