Nikkei has reported that Super Smash Bros. Ultimate has sold more than five million copies in its first week on the market. It mostly lines up with what we've been hearing the last couple of days as all regions reported on their individual figures. According to the Japanese newspaper, this includes both physical and digital sales.
A more important piece of information is that Ultimate might have outsold Super Smash Bros. for Wii U. That game has sold 5.35 million units, so it stands to reason that it was outclassed in its second week. Nintendo hasn't confirmed that piece of information, but the situation seems pretty cut and dry.