Grazie,
perfetto e sembra funzionare!

Ho modificato leggermente ancora cosi:

codice:
SELECT A.idAgency, 
           A.nameAgency, 
           A.dateAgency,
           COUNT(C.idAgency) AS totalCustomersAgency
FROM agencies A, customers C
WHERE (A.idAgency = C.idAgency) AND (A.idAgency = 36)
GROUP BY A.idAgency
Grazie mille!