I think 19 million will be a safe bet too.
I'm expecting holiday 2019 to essentially be flat, or just slightly down with just Pokemon Gen 8. But I expect Animal Crossing to come out in late June or July and just about single handedly raise the Switch's baseline well above what it was from July to October 2018. If we get a new SKU/model or price cut that makes the Switch more affordable for handheld gamers, then I think it's an even safer bet.
Hardware Comparison Threads:
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020