CrazyGamer2017 on 04 May 2018
This may be a bit confusing so I'll count from day of release so in my opinion sales of the Switch from beginning of March 2018 to beginning of March 2019 should be something around 10 - 12 million units.
So adding the 15 million units sold during the first year (March 2017 to March 2018) by March 2019 the total sales should be of 25 to 27 million units.
