https://www.gamespot.com/articles/forza-horizon-4-is-off-to-a-fast-start/1100-6462451/

Forza Horizon 4 shifted into high gear for a strong first week. Microsoft announced on its website that the game reached 2 million players in its first week of availability on Xbox One and PC.

The company did not provide any details on how this figure breaks down between copies sold outright and people who played using their Xbox Game Pass subscription. We also don't know if the figure includes people playing the free Forza Horizon 4 demo.