Pretty sure Wii shipped 25.95 its second (full) fiscal year. That was also the system's peak, it would be all downhill after that, dropping to 20 the next fiscal year, then 15 the following year, and then finally faltering to 9 mill.
As long as Switch stays on pace with the PS4 more or less, that's fine, everything else is gravy. Wii was driven by fad-like trends, Switch is a more sustainable type of system that may not peak as high but won't be held at the mercy of a wishy washy audience that craters later on. 3DS also took a hit in its middle years due to not having enough differentiation from the booming smartphone gaming market. Switch can play console-style games so that will always keep it cushioned from the impacts of mobile gaming.
PS4 is going to crush 100 million easily and probably hit 120+ mill ... long term that's a better platform to aspire towards, the challenge for Nintendo is going to be to not have heavy declines in year 4/5/6, which I think can be mitigated by a Switch Pro model. But PS4 did 17.7 in its second full fiscal year, Nintendo should be able to match/exceed that, if they can do even 18.5, that's a good result.
I got 20.77 from these numbers; https://en.wikipedia.org/wiki/Wii_sales
1st Full Fiscal Year of Wii Sales April 1st 2007 - March 31st 2008: 15.18 million (24.45-9.27)
2nd Full Fiscal Year of Wii Sales April 1st 2008 - March 31st 2009: 20.77 million (50.39-29.62)