I feel like it's way too early to give an exact answer; we still don't know its price or software which will be major factors, or how long it's life will be.
Just going off the hybrid form factor and expecting the usual Nintendo franchises and third party support equal to the current Switch, I'd say 120 million is the floor, but if it has some unexpected breakout hits, better-than-Switch third party support, an affordable price point, and lasts as long as the current Switch, I'd say 150m plus is on the cards.