Last number we have is Switch = 8.7M lifetime as of the end of November 2018, up from 4.8M by December 31st 2017.The three month after that were about 800k, so March 31st 2018 was approximately 5.6M, with the fiscal year being 5.6M - 900k = 4.7M, and the period from April 1st 2017 through December 31st 2017 is 4.8M - 900k = 3.9M. Current sales for the fiscal year is 8.7M - 5.6M = 3.1M as of the end of November 2018. Adding a 2.5M December to it would make the US have a 5.6M units sold between April 1st 2018 through December 31st 2018. That would mean that Switch is up for the first 9 months of the year by (5.6M-3.9M)/3.9M*100% = 43.6% up YoY. Adding a 2.25M December would give a 5.35M units sold between April 1st 2018 and December 31st 2018, meaning that the Switch would be (5.35M-3.9M)/3.9M*100% = 37.2% up YoY. The target for worldwide is to be 33% up YoY, so that would put the US well ahead of its personal goal. Furthermore, the 43.6% up number could go a long way to help other regions that might not be up 33% YoY. And lastly, these numbers are sold-through, not shipped. Shipments would naturally be larger since stock, while fully available, was not as present as it would be when there are no stock issues worldwide anymore.
You're right about shipments, of course: Switch remained sold out throughout 2017, and a couple of weeks into 2018, so shipments are approximately equal to sell-through. This year, that's not the case, so shipments will be noticeably larger simply due to the fact that supply isn't constrained anymore. That alone makes the situation for Japan (and worldwide in general, although the difference will be less pronounced because Europe and US didn't have stock issues anymore by the end of the year - but there will still be a difference I think, as I explained in the first part of this post) a lot better, and makes this shipment target of 20M very doable.
