Why? Is there a game you anticipate coming that day?
To predict 20 million in 2018 fiscal year, Nintendo might have planned great things. I dont need to antecipate exactly what these great things are, but by nintendo forecast, they exist.
But even if Nintendo forecast is too optimistic (what I think it is, Id bet in 18 million) I would put switch above N64 in december. Nintendo wont sell 5 million on Q4 2018 (might sell between 3 and 4 million), so ending with 2018 fiscal year anywhere between 35 and 38 million would make the 32 million mark to be crossed in Q3 ending (december).