With Bandai Namco recently confirming that Tekken 8 hit 3 million copies worldwide around it's anniversary at the end of January 2025, that provides strong evidence that FF7 Rebirth hit at least that number on PS5 given it ranked higher than Tekken 8 in the YE sales charts in the US and comfortably higher in Japan in 2024.
With PC game sale estimations on Gamalytic and VG Insights putting it somewhere between 469k and 959k respectively after 5 weeks, it's possible that Rebirth could be quite close to 4 million copies sold right now. Not great, but somewhere between 5 and 6 million before Part 3 releases in 2-3 years wouldn't be out of the question, especially if it gets an Xbox and Switch 2 release.