How many customers did not rent material at Mike's store?
not at Mike's store refers to staff.first_name ! = 'Mike'
SELECT COUNT(T1.customer_id) FROM customer AS T1 INNER JOIN store AS T2 ON T1.store_id = T2.store_id INNER JOIN staff AS T3 ON T2.manager_staff_id = T3.staff_id WHERE T3.first_name != 'Mike'