From data released by Media Creates, Mario Kart 8 Deluxe has sold over 1 Million units, more specifically 1,045,438 units, just in Japan alone. This is just incredible considering that Mario Kart 8 Deluxe is just a repackaged port of Mario Kart 8 on the Wii U with subtle feature changes like 2 item holding and DLC inclusion.
In fact, Mario Kart 8 for the Wii U itself also sold more than 1 Million units in Japan, selling 1.36 Million In its lifetime. Therefore it is not hard to conceive that many bought the game again on the Switch. It is also possible for Mario Kart 8 Deluxe Sales to eclipse the Wii U Sales in less than a month going at this pace.