Were aren't enough Switches around for this first month number.
Why not? Switch is at least at 5m by now, and this will greatly increase once Splatoon 2 launches. After it's 1 month on the market, I bet Switch will be around 6m, so 3m first month (4 weeks) should represent a 50% attach rate. MK8Deluxe sold around 2m first month according to VGC, on a lower install base while at that, so while 3m is not garanteed for Splatoon, it's not that hard either.
Bet with Teeqoz for 2 weeks of avatar and sig control that Super Mario Odyssey would ship more than 7m on its first 2 months. The game shipped 9.07m, so I won