Too Long; Didn't Read
The Yakuza series has very scarce information on the sales numbers of official games. From what we could gather, here's the top 10 based on sales:
10. Ryu ga Gotoku Ishin!
9. Yakuza 4
8. Yakuza 5
7. Yakuza: Like a Dragon
6. Yakuza 3
5. Yakuza 6: The Song of Life
4. Judgement
3. Yakuza 2 (and Kiwami 2)
2. Yakuza (and Kiwami)
1. Yakuza 0
This listing might be inaccurate, as many of these games found new success long after their release - a fact apparent by the series achieving 17 million sales as of 2021.