According to the recent report by Sensor Tower, the total installs for Mario Kart Tour including iOS and Android amount to 20 million within the first 24 hours while grossing at $1 million globally.
While 1 million may seem a lot, it has not yet included the revenue from the Gold Passes which includes a two-weeks trial.
Whatever it is, Mario Kart Tour is definitely the biggest potential cash cow for Nintendo right now thus expect more regular updates for the game!