Don't be fooled - in this day and age there's absolutely no reason that Apple can't almost 100% accurately know how many they have on order and how many they can ship. .
No one expected the 5s to sell over 2:1 over the 5c. Apple are some of the best at predicting demand for mature product categories, but anyone thinking that a company will know how many devices to manufacture on day one for an entirely new product category is just talking hot air.