At least 50 000 people have bought the game on DLsite and Steam combined. Maybe closer to 100 000. The russian and chinese fanbases are huge and I can tell you they've pirated the game. Hell I think most english speaker pirated it too. I'd estimate 500 000 to 1 000 000 people have played Black Souls. Fucking million.
Meanwhile indie devs churning out run of the mill Slay the Spire clones and moody art deco get 100 to 1000 sales if even that.