9, because it's a hybrid and a successor to both the Wii U and 3DS. Handhelds like the DS and 3DS are considered 7th and 8th generation respectively despite releasing more than a year before the first home consoles of the gen. The Switch's successor will make things muddy again if it releases years before an PS6 and Xbox 5. Sometimes generations are difficult to pin.
Lifetime Sales Predictions
Switch: 151 million (was 73, then 96, then 113 million, then 125 million, then 144 million)
PS5: 115 million (was 105 million) Xbox Series S/X: 57 million (was 60 million, then 67 million)
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima