lol to the one person so far who voted 15 - 17.5 million! The 17.5 to 20 mil option should have been the lowest, as anything under 20 million is seriously lowballing it. I bet 22.5 to 25 mil as I think Switch will decline a few million from this year. It'll probably hit in the upper part of the range though, maybe just under 25 mil.
Switch should actually have the supply to meet demand next year, unlike this year, since they've increased production so much this year. And they've got new model, BotW2, likely price cuts or at least for the original, a big pokemon remake, Mario 3D World port, MH, likely stronger than ever 3rd party, and probably a bunch of other great 1st party games. On paper you'd think it would beat 2020 but I don't see beating AC + Covid demand, plus you've got some competition from the next gen systems that'll factor in a bit to where people spend money.