Who are the customers?
select customer_details from customers
Which customers have more than 1 policy?
select t2.customer_details from policies as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t2.customer_details having count(*) > 1
Who has the most?
select t2.customer_details from policies as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t2.customer_details order by count(*) desc limit 1