How many accounts are there?
select count(*) from accounts
How many accounts does each customer have?
select customer_id, count(*) from accounts group by customer_id
Which customer has most accounts? Show its id.
select customer_id from accounts group by customer_id order by count(*) desc limit 1
Please also show the number of his accounts.
select customer_id, count(*) from accounts group by customer_id order by count(*) desc limit 1