When Smash 3DS came out in September 2014 in Japan, it sold ~950k units physically, so with digital it surely exceeded 1 million units at launch, on an 3DS install base of roughly 16.2 million.
When Brawl came out in February 2008, it sold ~842k units on a Wii install base of roughly 5.2 million.
Right now, VGC has the Switch install base in Japan at a little over 4.3 million. By the time Smash Bros. Ultimate comes out, it should be close to the 6 million mark at an average baseline between 45-50k.
Usually, I'd say that the difference in install base will limit the sales potential at launch, but it didn't seem to affect Kirby Star Allies or Super Mario Odyssey. And since it's launching right at the peak of the holiday season, I'm expecting around a 1.2-1.5 million launch weekend, and by the end of December, I think it will be at 2 million.