Which city has the most number of customers?
SELECT T2.city FROM Customers AS T1 JOIN Addresses AS T2 ON T1.customer_address_id  =  T2.address_id GROUP BY T2.city ORDER BY count ( * )  DESC LIMIT 1
most number of customers refers to MAX(COUNT(customer_id)); city with the most number of customers refers to grouping by city and ordering by the count of customers in descending order;