How many different clients have rented materials from Jon Stephens?
'Jon Stephens' is a full name of a customer; full name refers to first_name, last_name;
SELECT COUNT(T1.customer_id) FROM rental AS T1 INNER JOIN staff AS T2 ON T1.staff_id = T2.staff_id WHERE T2.first_name = 'Jon' AND T2.last_name = 'Stephens'