13 million of them were sold on PC.
Popular farming simulator Stardew Valley had sold over 20 million copies by March 2022. According to the game's website, 13 million of them were sold on PC, with the rest split between multiple platforms Stardew Valley supports.
In September, the game reached 15 million copies sold, so such growth is certainly admirable for a game that came out in 2016 and was developed by one person.
The creator Eric Barone, also known as ConcernedApe, is now working on Haunted Chocolatier – an RPG/simulation game where you gather ingredients and make chocolate.