I'll go with 15 million

15m easily.

But Subspace sucked.
The concept was good, and it was a decent adventure... until you got to the Great Maze. That just completely killed it for me.

However, I do think adding an actual single player Adventure Mode campaign of some sorts would certainly help.

So, if it's both; 36 million? :P

10-12 million.

This could actually be possible. Super Mario Odyssey did 9.07 million between from its release to December 31st.
Granted, Odyssey had an extra month to work with and ~1 million short, but I remember Odyssey wasn't didn't even crack the Amazon Rankings until after the September Direct where we get shirtless Mario :P. Smash has been hanging around those rankings ever since the first teaser back at the March Direct and it shot right to #1 after E3. If this holds true for other retailers and other regions. Good Lord, December is going to be massive.

Probably a bit more than Brawl, so 14-16 million.

Wanna take a bet that it will do 8m on December alone?

15 million. Smash has grown with every entry. Smash 4 is tricky to talk about but I think even with that game the Smash series only got more popular.

Yeah factor in that by the end of June NS will likely be around the GC mark as sales go so come December the userbase will be more than enough to snap up those numbers and this will be a few weeks afters the phase 1 of Pokemon as well, Smash and Switch will be the go to Christmas presents as well for a tonne of people.