I spent the last two months identifying all of the Nikens available for sale in the U.S., as found on Yamaha's website, on Cycletrader.com, and on Motohunt.com. I purchased a brand new 2019 Niken GT for my wife and then a brand new 2019 Niken for me.
In the process, I accumulated just under 50 VINs from Nikens for sale, about 3 from salvage auctions, a couple from your posts with other members, and one from an old article, for a total 54. I noticed that they were all 2019 models. Since there are 2 dozen never-sold 2019 Nikens sitting on showroom floors today and no 2020's listed anywhere, including the Yamaha main website, it is apparent that Yamaha chose not to put any 2020's on the floor with all of the unsold inventory.
And since I managed to get a sample of about 20% of the VINs and none of them are for 2018 models, I don't believe there were likely any sold in the U.S. for that model year. One manufactured in August of 2018 was presumably for the 2019 model year.
I also noticed that all of the under-170 VIN numbers are Niken GT's and all of the over-300 VIN numbers are Nikens. I have roughly every 4th VIN for Nikens and roughly every 6th VIN for GT's, along with where they are for sale, or were for sale, in the U.S. in the last two months. Since none of the ones I found are between 169 and 307, either (1) Yamaha produced only up to about 170 GT's in their first run and left room to produce more if they needed to later, or (2) the ones that are numbered between 171-300 were shipped to another country. Same goes for the standard model Nikens that ended at 410 - anything above that went elsewhere or Yamaha may have left a gap after 410 to have headroom for another production run.
Then I hit the NHTSA's website for the Yamaha recall on the brake switches. It gives the VIN ranges for affected Nikens, which confirmed the approximate numbers I had assumed from my list of 54 VINs. Look on the second page at VIN Range 3 (Niken GTs) and VIN Range 4 (Nikens). It shows #5-#174 inclusive (total of 170) and #310-#410 inclusive (total of 110): https://static.nhtsa.gov/odi/rcl/2020/RCLRPT-20V814-4925.PDF