Great sales. How is it doing in comparison to the 3DS when aligned? At first I didn't think the Switch could go past 20m in Japan even though the 3DS did, since it's half portable half home console, but now I can see it happening.
Also, the switch has been consistently outselling the PS4 in the US as well if I'm not mistaken, and sales in Europe are great too (even though they haven't reached PS4 levels).
Is it possible that the Switch will ultimately outsell the PS4 lifetime?
In Japan Switch is about 3m behind the 3DS, but the early price cut of the 3DS pulled sales that would have happened in later years forward, so it has a sales curve that is daunting in the first three years and tapers off afterwards. Switch might never top the 3DS's best year (~5.5m if memory serves right), but it has very good chances to post more 3m+ years than the 3DS due to better spaced out price cuts and revisions. It might take until year 7 before Switch matches the 3DS, but at that point the 20m mark would have already been crossed.
In the USA Switch is about 2m ahead of the PS4. Worldwide through 24 months - so both consoles have had each month of the year exactly twice - Switch leads the PS4 by about 4m units, an increase of 1m units compared to the period of the first 12 months.
So in the sum, Switch's deficits in Europe and the rest of the world are more than offset by its advantages in Japan and the USA. It is certainly possible that Switch sells more units than the PS4 lifetime, especially because of Switch's potential to sell multiple units to the same household, something that won't widely occur before the console is sold for a sub-$200 price.