We are almost a week exactly to the launch of Super Smash Bros. Ultimate and since the titles release Smash UItimate has already taken 1st place as the Nintendo Switches best-selling title.
According to Famitsu's early sales report for Japan, Super Smash Bros. Ultimate has already reached 1,220,535 physical units sold in a range of only about 5 days. To put into comparison, Smash Ultimate has already beaten Pokemon: Let's Go, Pikachu! and Let's Go, Eevee! as those titles have collectively sold 1,012,247 units.
To give an even better comparison, Nintendo's The Legend of Zelda: Breathe of Wild was released back in March of last year and has totaled 1,182,281 units. Smash Ultimate has already surpassed that, solidifying it as Nintendo's best-selling Switch title. There is more statistics on Smash UItimate sales from the UK sales charts, check that out here.