At this moment, it should be somewhere between 4-5 million.
By the time we get Nintendo's next report in January, it should easily and comfortably be within the top 3 alongside Mario Kart and Breath of the Wild.
I think it will ultimately go on to sell around 13-15 million and be the best selling 3D Mario game.
Hardware Comparison Threads:
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020