How many became customers before that?
SELECT count ( * )   FROM customers where date_became_customer <  ( SELECT t1.date_became_customer FROM customers AS t1 JOIN customer_orders AS t2 ON t1.customer_id   =   t2.customer_id JOIN order_items AS t3 ON t2.order_id   =   t3.order_id GROUP BY t1.customer_name ORDER BY sum ( t3.order_quantity )  DESC LIMIT 1 ) 
count of customers who became customers before a specific event refers to using COUNT(*) with a condition on date_became_customer; the specific event in this context is the date on which the customer with the highest sum of order_quantity across all their orders became a customer;