This week Ring Fit Adventure has surpassed 800K sales for 2020 on Famitsu. Game is still sold out and reselling for 75% more on Amazon.co.jp, even with a big shipment next week we are still nowhere near meeting demand. The best selling game in its second year on the Switch Splatoon 2 had sold 791K at the same time in 2018, Splatoon 2 never faced such shortages outside of it's launch and mainly due to the lack of Switch stock. Splatoon 2 finished 2018 with 1.12M something Ring Fit won't have any issues surpassing by the end of this quarter with the biggest hardware weeks still to come.
Wii Fit is the target with it's 3.56M sales in Japan, maybe by the end of 2021 if Nintendo doesn't launch a sequel for Ring Fit we would see it surpass Wii Fit's total in Japan.
I did check upon Wii Fit too and then saw your comment. And yes, by end of the year Ring Fit should already break the 2M mark, and at the speed it's running, it should pass 3M before the end of 2021.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454
The Steam/GOG key gifting thread: https://gamrconnect.vgchartz.com/thread/242024/the-steamgog-key-gifting-thread/1/