Ok let me try (its gonna be cool to bring this thread back from the dead in 5 years from now and see how many of use where wrong or right).
January 2018 = 74 million (19 million in 2017)
January 2019 = 91 million (17 million in 2018)
January 2020 = 105 million (14 million in 2019)
January 2021 = 113 million (8 million in 2020) This is when the PS5 is anounced (in february) and released in the holidays, just like the PS4.
January 2022 = 118 million (5 million in 2021)
January 2023 = 120 million (2 million in 2022).
Total Sales 121 million