I'm thinking 10-15m, momentum generally the same (or really a little less).
Nintendo had an extremely good first party line-up it's first year: Mario Kart, 3D Mario, and the best Zelda in several decades, plus several other 1st party titles. Nintendo has Pokemon still, but I don't think Pokemon is coming til 2019. The leap from 3DS to Switch means it will need a little extra development time. Switch will also have much better 3rd party support in 2019.
I don't think 2018 will be nearly as good as either of those years. It still has a couple of other strong titles for 2018 like Smash Bros or Animal Crossing, but I don't think that will be enough to really increase momentum over 2017. Also Nintendo is releasing it's online service this year. I expect it will have some kinks that will have to be smoothed out. I expect the service to be good eventually, but Nintendo seems out of their water with anything involving the internet.
So a combination of less impressive first party games and an initially disappointing online service will kill a little of the Switch's momentum.
curl-6 bet me that PS5 + X|S sales would reach 56m before year end 2023 and he was right.
My Bet With curl-6
My Threads:
Master Thread, Game of the Year/Decade
Switch Will Be #1 All Time
Zelda Will Outsell Mario (Achieved)
How Much Will MH Rise sell?
My Bet With Metallox