De acordo com o diagrama do esquema de banco de dados relaci...

SELECT r.nom_regiao, m.nom_municipio,
COUNT(a.id_corretor) AS total FROM
MUNICIPIO AS m INNER JOIN REGIAO AS r
       ON m.id_municipio = r.id_municipio
RIGHT JOIN ATENDE AS a ON r.id_regiao = a.id_regiao
GROUP BY r.nom_regiao, m.nom_municipio

SELECT r.nom_regiao, m.nom_municipio,
COUNT(a.id_corretor) AS total FROM
MUNICIPIO AS m INNER JOIN REGIAO AS r
       ON m.id_municipio = r.id_municipio
LEFT JOIN ATENDE AS a ON r.id_regiao = a.id_regiao
GROUP BY r.nom_regiao, m.nom_municipio

SELECT r.nom_regiao, m.nom_municipio,
COUNT(*) AS total FROM MUNICIPIO AS m
INNER JOIN REGIAO AS r
      ON m.id_municipio = r.id_municipio
INNER JOIN ATENDE AS a ON r.id_regiao = a.id_regiao
GROUP BY r.nom_regiao, m.nom_municipio

SELECT r.nom_regiao, m.nom_municipio,
COUNT(*) AS total FROM MUNICIPIO AS m
FULL JOIN REGIAO AS  r
       ON m.id_municipio = r.id_municipio
FULL JOIN ATENDE AS a ON r.id_regiao = a.id_regiao
GROUP BY r.nom_regiao, m.nom_municipio