At the end of chapter 6 there are 3 characters at the bottom of the page, つづく. If I'm not mistaken, it basically means "to be continued". I believe it's just volume 1. I also believe there's a novel as well but it hasn't been translated. I'm almost sure I saw it at one point.
If anyone has more information or if I'm wrong, please correct me :3
I might have missed something but I just finished reading chapter 19 and moved to chapter 20 but it really doesn't really make sense. Are and of the chapters missing?