To be more specific, 1,238,358 retail copies in Japan were sold in the first three days starting from the 7th of December, which was when the game launched, and the 9th of December. This is also said to be the highest opening week for a Switch title, and also the highest opening week for the Super Smash Bros. franchise.
It also seemed like the game managed to spur additional Switch console sales because the report claims that 278,313 units were sold that week, versus the 107,450 units that were sold the previous week. Given that the Switch is the only console that Super Smash Bros. Ultimate is playable on, we suppose gamers have had little choice if they wanted to play it.
Nintendo also has plans to release additional DLCs for the game that will introduce new characters, stages, and soundtracks, which we imagine will encourage gamers to play more.