>>281569484
The first Hand Demon could serve as a kind of tutorial boss like the Asylum Demon, and there are about eight named demons below the Lower Moons from Season 1.
Then you add in the six Lower Moons, six Upper Moons, plus Nakime, Kaigaku, and finally Muzan as the last boss, and that brings the total to 23 bosses.
If you separate Daki and Gyutaro, or Hantengu and Zohakuten, you can pad the count even further.

Even then it might still feel a little on the short side, but that’s just the nature of anime games.
It would still be enough to work perfectly well as Kimetsu Souls.