Red Dead Redemption 2 has sold-in more than 57 million units globally to date. That's up 2 million since last quarter. Franchise is at 81 million units, compared to 79 million last quarter.
As someone who loves the first game and didn't like the second one at all, I'd say: just be sure you know why you like the first game. The second is quite different. To me, the second is more like a movie than a game, with huge, slow intermissions.
I liked the second game but can totally understand why some wouldn't. I'd say the first game is a western action game, while the second game is more like a cowboy sim. I find it much more immersive, but much slower paced.