How many did he purchase?
SELECT sum ( t3.order_quantity )  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
how many did he purchase refers to SUM(order_quantity);