do you know how many total customers had more than 1 policy?
SELECT count ( * )  FROM  ( SELECT T1.customer_details ,  T1.customer_id FROM Customers AS T1 JOIN Customer_Policies AS T2 ON T1.Customer_id   =   T2.Customer_id GROUP BY T1.customer_id HAVING count ( * )   >  1 ) 
total customers with more than 1 policy refers to counting the number of customers after grouping them by their customer_id and filtering those who have a count of policies greater than 1.