Due to a typo Nikonrumors has corrected this number to 3500 per month, taht means approx 10 years to meet a request of 400000 units
https://nikonrumors.com/2021/11/06/update-the-monthly-nikon-z9-production-is-3500-cameras.aspx/
It seems likely that the 400k figure is just someone's wild fantasy. These expensive high-end bodies don't sell in large numbers. The 3500 per month seems realistic as I recall 8000 for D3 and 80000 for D300 once upon a time.
I simply do not believe that 400 thousand people are willing to pay six thousand euros for a single camera body. What might be possible that 400k people would buy a follow-up model in the 2000-3000€ price class in the form of a Z6 III or Z7 III with Expeed 7 giving some of the AF benefits trickled down from the Z9. I look forward to that happening.
However, in any case, the Z9 will likely improve Nikon's brand and restore faith in the future in the system and also in part restore the second hand market value of many F-mount lenses as it has been shown that the Z9 autofocuses these better than ever, at least in daylight. Hopefully also in low light the performance will be good. I am glad that I've been stubborn in not even considering the sale of my 200/2 and similar lenses and instead have strengthened my set of fast F-mount lenses. Now it seems that these will have lasting value for years and even decades to come, even in the situation that the majority of cameras sold are mirrorless in the future. In shorter focal lengths I can see the optical benefits of Z mount, for example, the 35/1.8 Z S is quite a wonderful lens. But I feel the F-mount fast telephotos are about as perfect as lenses come, and I quite like the image aesthetics from them. Yes, there will be improvements in the future and better focusing and VR systems, but I no longer think that a lot of these excellent lenses will collect dust, instead they will continue to be put into use.
This return of brand value likely helps Nikon sell a lot of less expensive products and hopefully much of the Z9's power can be implemented in lower-cost bodies that the majority of photoraphers can eventually (hopefully soon!) enjoy.
If it did turn out true that they have 400k preorders then they would need to find a way to increase production capacity dramatically (assuming the 3500 per month is true); I find it hard to believe people would wait for a decade or more for delivery.
