It did it faster than Forza Horizon 4 reaching the 10 million players milestone.
Forza Horizon 5 was released back in November and turned out to be a major success for Playground Games and Xbox Game Studios. It performed better than it's already very popular predecessor already from the start, and has continued to climb after that.
Yesterday it was noticed by several eagle eyed gamers (like Benji Sales on Twitter) that the score board now reveals that over 20 million players have raced through Mexico. As a comparison of how much this is, Forza Horizon 4 needed eight months to reach ten million players, while Forza Horizon 5 has hit over 20 million in just seven months.