Nintendo has already sold over two million copies of Super Mario Odyssey within the first three days of its release. That can be deduced from Nintendo's Six Months Financial Results Briefing.
One of the slides included in the briefing PDF shows that the company has already sold over two million copies of the game. The game was released last Friday.
In Super Mario Odyssey Mario has to keep Princess Peach from Bowser's claws, who want to marry the princess. To do this, Mario has to travel through a number of different locations, including New Donk City, a copy of the non-fictional city of New York.