count the lessons taken per customer first name.
select t2.first_name, count(*) from lessons as t1 join customers as t2 on t1.customer_id = t2.customer_id group by t2.first_name
How many lessons did customer with first name Ray take?
select count(*) from lessons as t1 join customers as t2 on t1.customer_id = t2.customer_id where t2.first_name = "Ray"