Previously, it was the fastest-selling title in the series but it now has become the best-selling in the franchise's 17-year history. Kingdom Hearts 3 was the first mainline entry in the series since Kingdom Hearts 2 released in 2006. Between that time, publisher Square Enix released entries in the series such as 358/2 Days, Birth By Sleep, and Dream Drop Distance
Source: https://www.dualshockers.com/kingdom-hearts-3-best-selling-game-franchise-history/
I'd say 8-10m lifetime. Very good achievement for square enix