Why does it say chapter 12 when it's only the 9th? Or are some missing here?

Dont be bothered. no missing chaps. Just that the release of chaps in manga are different in magazine. This is how it works:
ex: 2 chapters are released (15 pages per chap) in magazine. that 2 chapters were merged into 1 chapter (30 pages) in manga.

Thus making the magazine chapter count more than the manga.