Im not sure the Switch can sell 120m units based on its current tarjectory.
Particularly, is there an audience of 100-120m people who want to mainly play Nintendo's games? Their only consoles to go over 100m relied heavily on appealing outside of their core userbase (wii/DS) or being the only big player in the handheld market (Gameboy).
I'm inclined to think the Switch will end around or below the PS4 with a steep decline in later life.
Sure, one of the main issues with Nintendo has been that their audience is split between their handheld and home console ecosystems. On top of that Nintendo software teams have had to split resources between 2-4 different platforms.
For example, in 1999/2000 Nintendo was working on late GBC & N64 games while also working on early GBA & GC games. In the 2004-2006 period they would have been working on late GBA & GC games while also working on early DS & Wii games then around 2009-2012 they had to work on late DS & Wii games while also working on early 3DS & Wii U games.
That means each generation only gets a few years of Nintendo's full attention and even in those years support would be split between handheld and home consoles. This progressively has gotten worse as development costs and times increase.
Switch is the first time since the mid 80s that Nintendo gets to give 100% of their support to a single platform which means their audience will all be in one place and sales will not plummet or stagnate like many previous devices have.
I'm not saying it will pass PS4 but 100-120 million is not out of the question at all.