I've been on DPReview's Nikon forums since 2005. I have seen exactly the same set of posts as we see this year with the z9: Promises, guesses, intuition, WAG's, and nay sayers abounded.
In that almost 17 year period, the common thread has been that Nikon does not give much advance notice to its dealers about the availability and quantity of high end cameras, and at the same time there have been people claiming insiderknowledge, someone who answered the photo of big dealers, and/or "educated guesses." I've yet to see accurate advance notice advanced by dealers of more than a few days.
I started with a D2x, which had moderate delays, but I got mine from a local dealer after a short time [I no longer recall how long, but think it was about a month, and I was late ordering]. Since then, almost always in the first month of delivery starting, I've bought a D3, D300, D800, D810, D850, D4, and quite a few lenses. My experience has been that every camera and every lens was delivered in perfect condition, no failures, and every one performed as promised.
When I used to.have a local dealer [a Calumet branch], as soon as a camera was announced, I'd walk into the store and ask if they had a list started. With one exception, I was first or second on the list, and for that I was number 5. They would eventually get a call from national telling the store manager how many cameras would be allocated to it; they sat in Chicago until the local store confirmed the purchase: The local manager would call me and ask if I was ready - I always was, due to too many credit cards. I'd stop at the store on my way home, give my credit card and in a couple of days Calumet would ship the camera to the store for arrival. The one time I was number 5, only 1 person ahead of me had the money or credit.
Since Calumet closed down, I've used B&H. Always the same routine as we see here. The one thing I learned is to watch the forums for real information about when the store would open for customers: Get up early, dial repeatedly to get through, and place the order. No other plan ever seemed to help anyone get accurate info. One time, I forgot to set an alarm and didn't place my order until the afternoon - that time, I had months of waiting - I think it was for the D850.
Never have NPS members gotten all the cameras shipped the first day - there have always been something going to the rest of us. That's why I set my watch to vibrate 15 minutes before 8:00 a.m. so I could be ready. I
f I get a camera soon, I'll post the info so that a database can build about how early you have to order to get a hot ticket item.