Wii U userbase is obviously shit, which is less of a deal for Nintendo's biggest games than for other consoles, but still important. And a lot of those people who are big Nintendo fans will be buying it on Switch. So how much will it sell on Wii U? I think 1-2m.
Then it is dependent on how successful Switch is. I think it could sell over 1m first week. It is difficult to say how much Switch itself will sell first week, but I think it should be over 1.5m, and for this game, especially given it's lack of competition, a 66% (or so) attach rate isn't ridiculous.
Then it is down to legs on the Switch. This will be pretty dependent on how successful the console is going forward. Zelda will, I imagine, be the most popular purchase overall when people buy new systems up until Mario in November (Splatoon 2 in Japan). I would guess it will be 5m-6m by the end of the year. It may have decent legs past that point given it's quality and name, and maybe even some double purchasing from Wii U owners coming forward a gen.
I will guess 8m lifetime