They sold 9 million iPhones (of all types) in the opening weekend of the 5s launch.
20 million is doable but that would be all phones not just the Plus. Given that half or more want the smaller one that would assume the demand is there for 40-50 million total iPhones for launch weekend.
Ain't going to happen, though I'm sure it would be nice for their stock. Though if you're right, I will eat crow.
Though again keep in mind we have no idea how long they've been pre-manufacturering. Chances are Foxconn did less units daily last time and they got enough to sell 9 million. So given their increases in labor maybe they do have enough for 20+.
I don't think manufacturing started 25 days before launch (late August). I think it was more like June, and probably just shelved units to load OS when it was good mastered and box it.
Oh, I wasn't specifically focused on launch day or even week. But I do believe that much demand is latent and is accessed once the lines die down and the shopping experience becomes more pleasant. Lots of people would prefer to do this sort of purchase in person, with a phone in hand, and then a sales person to confirm that you've got the right one or signed up for the right long term two year contract. Those people want the iPhone but they don't buy during the first two months because of the hassle and their nervousness of just doing this through the mail.
So when you look at the 9 million which sold during the launch weekend of 5s that is just a FRACTION of the actual demand for that phone that weekend. Those were people willing to wait on lines for hours or to get up at 3 a.m. to place a pre-order for a phone that (a) they had never touched and (b) they hadn't even read a review on or known anyone who had ever used one. Think what a filter a buying process like that is. There were many more millions, I would suggest, that knew the stores had nothing. Or were turned away from the stores. Or didn't want to fight a line.
So in the mythical world where Apple has all the phones it can sell available and the staff to process so that there are no lines and the websites offer two day delivery from when you order even if you wait until to order until after the reviews have come out, then yes the past sales numbers would be blown away. But maybe not quite as high as selling 20 million 6+ in the US in a weekend.
I doubt they were really doing serious scale manufacturing in June. They have to test those prototypes a long time. And also, if 100,000s are being made in June, then I don't see why fully put together leaks don't come out then instead of the process where they become more and more available in late August.