If they wanted to they could manufacture enough so that everyone who wants one could have it within the first month. It isn't like they don't know what the demand for their products are.
Apple sold 78.29 million iPhones in Q1 2017. Someone on this thread mentioned that at full capacity Apple can ship 800,000 phones each day. So to meet the iPhone demand in 1 quarter, it would take almost 98 days to make all those phones, which is roughly 3 months.
The iPhone 7/7+ was released on September 16, 2016. So lets that they are cranking out 800k phones a day (unlikely because its a new product so they'd have to work out the kinks at first). They'd need to have all designs and testing done, as well as every single component ready by June 16, 2016 at the latest just to have enough stock for 3 months worth of product.
Lets go one step further and look at total iphone sales in 2016, which was roughly 215 million based on a bar chart I phone from business insider. Applying the same 800k phone yield to the 215 million sales in a year, it would take them 268 days to satisfy a year of iPhone demand, which is ~9 months straight. That's not very realistic.