We had heard that over at GameStop, the attachment rate for the game was pretty much one-to-one, meaning that for every Switch sold, a copy of the game was purchased as well. However it looks like maybe Mario Kart 8 Deluxe was a more highly-anticipated title because over on Amazon, the game has already managed to outsell Breath of the Wild.
This is based on Amazon’s “Best Sellers of 2017” list which is updated daily, and at this time of posting Mario Kart 8 Deluxe is in third place, with Breath of the Wild in fourth. Note that this is only for Amazon’s own platform, meaning that overall Breath of the Wild could still have sold more units.
Also it should be noted that Amazon does not count digital downloads so maybe Breath of the Wild is still in the lead. However it does show how many players have been looking forward to the game (which is already available, by the way).