Which city has 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
city with the most number of customers refers to MAX(count(customer_id)); joining Customers and Addresses tables on customer_address_id and address_id;