According to Steamspy, which used to be capable of giving more precise data but is not able to anymore:
Please note that this only includes Steam and not Epic purchases or in the case of FFXIV and FFXI, external launchers.
Final Fantasy - 500,000 - 1,000,000
Final Fantasy II - 200,000 - 500,000
Final Fantasy III - 200,000 - 500,000
Final Fantasy III (3D) - 500,000 - 1,000,000
Final Fantasy IV - 100,000 - 200,000
Final Fantasy IV (3D) - 200,000 - 500,000
Final Fantasy IV: The After Years - 50,000 - 100,000
Final Fantasy V - 200,000 - 500,000
Final Fantasy VI - 200,000 - 500,000
Final Fantasy VII - 1,000,000 - 2,000,000
Final Fantasy VII Remake - 500,000 - 1,000,000
Final Fantasy VIII - 500,000 - 1,000,000
Final Fantasy VIII Remastered - 500,000 - 1,000,000
Final Fantasy IX - 500,000 - 1,000,000
Final Fantasy X/X-2 - 500,000 - 1,000,000
Final Fantasy XI - 0 - 20,000
Final Fantasy XII - 1,000,000 - 2,000,000
Final Fantasy XIII - 1,000,000 - 2,000,000
Final Fantasy XIII-2 - 200,000 - 500,000
Lightning Returns: Final Fantasy XIII - 200,000 - 500,000
Final Fantasy XIV - 2,000,000 - 5,000,000
Final Fantasy XV - 1,000,000 - 2,000,000
Final Fantasy Type-0 - 200,000 - 500,000
World of Final Fantasy - 100,000 - 200,000
Crisis Core: Final Fantasy VII - 200,000 - 500,000
Final Fantasy Dissidia NT - 500,000 - 1,000,000