First, we can select the customer_number and the according count of orders using
SELECT customer_number, COUNT(*) FROM orders GROUP BY customer_number
Then, the customer_number of first record is the result after sorting them by order count descending.
In MySQL, the LIMIT clause can be used to constrain the number of rows returned by the SELECT statement. It takes one or two nonnegative numeric arguments, the first of which specifies the offset of the first row to return, and the second specifies the maximum number of rows to return. The offset of the initial row is 0 (not 1).
It can be used with only one argument, which specifies the number of rows to return from the beginning of the result set. So
LIMIT 1 will return the first record.
SELECT customer_number FROM orders GROUP BY customer_number ORDER BY COUNT(*) DESC LIMIT 1 ;