There's a chance Breath of the Wild 2 comes out next year, so on that alone it'd probably be hard to predict anything less than 20 million. Then again, I'm not particularly confident they will get the game out that quick, but still, it feels inevitable that they'll release some killer app for it next year. Don't think the next-gen consoles are gonna have an impact yet, Switch sales are only gonna start declining in 2022.
They are reusing a lot of assets and most of the gameplay elements, so it's hard to imagine they won't be able to get it done in 4+ years when they worked on the first one for 5 and 2 systems.
That said, I am hard-pressed to predict that the Switch will sell anything under 18 million. It all depends on Nintendo, really. They could do anywhere from 18-26 million next year, depending on the release schedule.