1) Firstly, the PC sales were included in that 20 million figure. I don't know if Sony ever detailed the specifics on the split between PS4/PC sales (though obviously the PS4 version will have the vast majority of that number).
2) I feel that, even with Elden Ring releasing and being a great game, Horizon Forbidden West will still do great numbers over its lifetime. Once again including the inevitable PC release in a year or two, I am thinking it will also reach 20 million units sold like its predecessor.
Bonus Question) I am absolutely loving Forbidden West so far. Graphics-wise, it is a massive leap over the first one. Note: I played Zero Dawn on a PS4 Pro on a 4K HDR TV, and am playing through Forbidden West on a PS5, in its Quality Mode, on a (newer) 4K HDR TV. Gameplay and story-wise, it is about where one would expect a sequel to be at. Everything is refined and made more enjoyable, and overall dialogue and story scenes are a step-up. But it isn't like it plays like a whole new game or anything, nor should it be. If you enjoyed the first one, you will absolutely enjoy this one. If you didn't like Zero Dawn, there isn't anything in Forbidden West that will change your mind.
For me, the world of Horizon and Aloy as a character represent some my most beloved ideas in all of gaming. I put 103 hours total into Zero Dawn and the expansion The Frozen Wilds, and so far have 45 hours into Forbidden West and am at ~37% complete in the game.