I think it's sold around 12 million copies now, which is really good, especially in comparison to Assetto Corsa Competitzione. But the regular Assetto Corsa does spread it's net far wider than Competitzione does, so there is a clear difference in appeal between the two.
However, despite the great sales since it's relaease in December 2014, the figures for the number of people playing are are interesting. There are only 2,620 people playing Assetto Corsa right now on Steam (which accounts for over 50% of it's players and sales).
Compare that to the appeal of say GT Sport which has sold around 9.5 million since December 2017 and currently has 185,000 players online (186,000 yesterday), there's still a huge gulf in current popularity despite how well Assetto Corsa has sold over it's lifetime.