Final sales number really depend of what strategy Sony takes with PS4 in its final years. It’s been more than 3 and a half years since the last price drop for the system (September 2016).

As comparison, the PS3 received it last price cut on September 2012 ($269), more than a year before the launch of PS4. Ps3 did around 8.2 millions in 2013. PS4 is doing 3.6 millions in 2020 (1 million more compared to What the PS3 was doing in 2013 in the same time gap), so PS4 can do a little bit more than 10 millions for 2020.
However, that depends of how much is the price cut Sony will do, $50-100 and when.

But, it would end around 130-135 million units.