How many accounts are there?
select count(*) from accounts
Which customer has least number of accounts?
select t2.customer_first_name, t2.customer_last_name from accounts as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t1.customer_id order by count(*) asc limit 1
Please also show his id.
select t2.customer_first_name, t2.customer_last_name, t1.customer_id from accounts as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t1.customer_id order by count(*) asc limit 1