The announcement from the developers mentioned that two million units had been sold in just 13 days. At the time of the article, they also said that they hit a concurrent player peak of 360,000.
That peak has already been broken according to Steam DB, showing 392,862 peak players for today.
Valheim was created by a small team of five people and is published by Coffee Stain. The game is currently in Early Access and has received many positive reviews.