Write SQL query to solve given problem: How many different clients have rented materials from Jon Stephens?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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'