At Nintendo's annual shareholder meeting notes (via Game Informer), the company announced that Mario Kart 8 had sold more than 2 million copies worldwide. That means approximately another 800,000 sold in June. It had already caught 20% of the Wii U install base at that point, so unless many more people have picked up Wii U systems it should now be nearing 40%.
During the meeting, shareholders also reelected various executives, including designer Shigeru Miyamoto and president Satoru Iwata. Iwata himself was not at the meeting, having recently undergone surgery following a medical issue that caused him to miss this year's E3 conference.