Going back to 2014, my order went in 34 minutes after pre-orders opened for the iPhone 6 and I still got it on launch day. At that time, I didn’t have an iPhone, iPad or MacBook so was ordering from my laptop. Tried for half an hour and as soon as I switched to Internet Explorer instead of Chrome, it sailed through.
Fast forward two years, upgrading to the 7 Plus, I didn’t imagine I’d have any issues. 8:03 and it was available only in silver and gold.
Who knows how it’ll go, but if I had to guess, the forums will be full of screaming people unable to access the Apple Store to order and 3-4 week shipping dates within twenty minutes.