Hard to guess when Sony is selling all PS5s they can manufacture. Let's say 14 million, a bit lower than PS4. Reasons:
PS4 2014 numbers include the japanese launch. PS5 numbers in Japan will be worse.
As usual, USA will be the main selling force early in the gen. PS5 will face stronger competition in said territory because this time Xbox isn't a complete disaster.
That being said, while Playstation + Xbox sales have been similar the last two gens they've become more frontloaded, so this could offset what I've said above.