I would guess somewhere between DS and 3DS, depending on what Nintendo reveal at E3. If they add Smash Bros and Pokemon Stars to their line-up for August-December 2017, then I can see that powering Switch to a really, really big Christmas quarter when combined with Mario Odyssey. I'd say year one sales of 13-14 million in that case.
If Nintendo have already played their hand, and the end of year line-up is Xenoblade 2, Fire Emblem Warriors and Mario Odyssey, then I think 9-10 million by the end of next March is more realistic. I really do think the combined selling power of Pokemon and Smash - on top of Splatoon 2, Mario Kart, Zelda, 3D Mario in the 1st year - would power Switch to a much more impressive year. It then becomes a big question of whether or not Nintendo can maintain those kind of sales levels in the years to come.