Instead of storing the number of items in the order, you could modify the query that is going to display the number of items by using the Count() function which will return the number rows.
Here is query example:
SELECT t1.OrderID, t2.DetailOrderID, COUNT(t1.OrderID) as tot
FROM ps3_orders as t1 INNER JOIN ps3_orderdetails as t2 on t1.OrderID = t2.DetailOrderID WHERE t1.OrderID = 1
Let me know if you have additional questions.