Show the order ids and the number of items in each order.
SELECT order_id ,  count(*) FROM Order_items GROUP BY order_id
SELECT order_id ,  count(*) FROM Order_items GROUP BY order_id
[{'order_id': 1, 'count(*)': 2}, {'order_id': 2, 'count(*)': 1}, {'order_id': 4, 'count(*)': 3}, {'order_id': 10, 'count(*)': 1}, {'order_id': 12, 'count(*)': 1}, {'order_id': 13, 'count(*)': 4}, {'order_id': 14, 'count(*)': 1}, {'order_id': 15, 'count(*)': 2}]