Hard to say. Breath of the Wild was at 4.70 million according to their reports by September 30th. I have no doubt Odyssey is easily 3 million +, probably over 4 by now with Black Friday and all but Zelda would most likely have sold another chunk as well with the holidays.
Optimistic side I'd say Mario is closing in on 5 million. Zelda should be at 5.5 or so by now? Possibly more, I'm never good at predicting :D
I would say that is pretty reasonable speculation. It’s hard to say because there are so many games to choose from when buying the Switch this first holiday season.
It is said the majority of people buy one or two games with a new console. The choice of SMO, BoTW, MK8 DX, (maybe Splatoon) makes that a tough choice. I am totally guessing (not factual) that most people get one single player and one online game. I am guessing that because that is what I do.
In a nutshell I think you could be right but SMO may be closer to 4m.