2 million in its first weekend, right before the holiday season. 

For comparison's sake, and these are VGC numbers, so take: (NA + Europe + Japan: Launches aligned)

Galaxy 1 - 526,910 + 153,922 + 260,993 = 941,825 units.

Galaxy 2 - 705,394 + 227,478 + 340,397 = 1,273,269 units.

3D Land - 302,878 + 143,853 + 341,904 = 788,635 units.

No 64 or Sunshine since there's no VGC data on them, and no 3D World for reasons obvious.

The games that I think are the closest to compare would be 3D Land and Galaxy 1, seeing as how those two games launched within the first year of their respective system's life cycles and went on to sell throughout their generations If VGC can keep it close, since I know we don't track digital and we count physical copies sold to consumers instead of shipped to retailers, Odyssey should have the best launch ever for a 3D Mario game. That we know of.

15 million at worst, no less than that.

Yeah, I doubt that...

I think fifteen million is possible (not probable, but not highly unlikely) but 20 million seems out of question.

How is 15 million not probable?

Odyssey is so far the fasting selling Mario 3D ever, and we already saw how successful some IP can be on Switch. This is just the same case of Zelda, where no game ever surpassed 10 million (the best selling was TP with 8.5 million) and now Breath of the wild is on the road for 10-12 million.

Anything less than 15 million, is very bold, considering Galaxy almost did 13 million, and Odyssey should easy beat him.

Not bold enough. Life time sales will be close to 30 million units. Owning this game will be a requirement any time a parent buys a switch for a birthday or Christmas.

All games have their saturation point. Just because it is the fastest selling within the current time interval (when people are starved for new Switch games, as the platform is pretty new and active) does not mean the trend will continue. There is a limit to the number of people 3D Mario appeals to, and I expect that to be comparable to Super Mario Galaxy's sales. Especially as the Switch has a much stronger library over time. Super Mario Odyssey might outsell galaxy, but I don't suspect it will do it by more than two million. Hence "fifteen million is possible (not probably, but not highly unlikely.) 

There is really no reason to think that just because it is the fastest selling (up until now) that it will continue to outpace the rest of the series at the current rate.  

Same thing for Breath of the Wild, although in BoTW's case the conventions were change much more significantly than in SMO's.

Breath of the Wild has a much wider appeal than other 3D Zeldas did. It did something different and interesting to expand the people interested in it. It also has social-media with thousands of people sharing to millions of others about how much fun they're having with Zelda. doing all this really cool stuff

Super Mario Odyssey doesn't have that advantage. As sc94597 above points out, games do have their saturation points regardless of how well hardware sells, or how fast the software sells. We've seen Final Fantasy games sell most of what they're ever going to sell in a 1 day period.

I feel like you say this in all my bold threads xD

Comparing FF to Mario in terms of legs was your mistake. If you think they Are comparable your surely mistaken