As seen on Nintendo’s financial results released today, Mario Kart 8 Deluxe has become the best-selling franchise title with 38.74 million copies sold.
With the latest info, Mario Kart 8 Deluxe has surpassed the overall sales of Mario Kart Wii at 37.38 million copies. The kart racing game first release in April 2017 has more copies sold than even the very popular title Animal Crossing: New Horizons (released on March 20, 2020) at 34.85 million.
The graphic also shows Mario Kart 8 Deluxe is the best-selling Nintendo title sold so far.

Mario Kart 8 Deluxe is an enhanced version of the Wii U title Mario Kart 8 (first released on May 30, 2014).
[Source]: Nintendo: Financial Results Explanatory Material – 2nd Quarter of Fiscal Year Ending March 2022 – [Archive].