The PS4 sold I believe
7.5 million its launch partial year (Nov-March 2014)
14.8 million it's first full fiscal year (Ending March 2015)
17.7 million its second full fiscal year (Ending March 2016)
2.74 million 1 month launch (March 2017 only)
15.05 million its first full fiscal year (Ending March 2018)
The way I see it anything over 17.5 is a pretty good success and keeping in line with the PS4, granted the PS4 had a few extra months because it launched in November instead of March. But if we're looking at full fiscal years.
20 mill is a nice number, but I don't think it's the be all, end all. Anything above 17 mill for the fiscal year indicates healthy Switch performance and is keeping in line with the PS4's sales at about the same age. What's more important is not having up and down quarters as much. You don't want so much riding on the holiday season, that's not how a successful hardware platform should operate, sales momentum should be a more consistent throughout the year.