About the adaptation…

SkullPlease19 said:
It is a 1:1 adaptation don't worry.

While the anime adapts 8 volumes, which are the full series, there is another volume called "Scum's Wish Décor" aka volume 9.

But this wasn't published directly after the chapters of vol 8 were out, it is merely a fan service volume for us the fans (mengo said so in the afterword of this volume), but it brings also a conclusion to the series and shows the characters after a Timeskip (don't remember how long the Timeskip is but definitely a couple of years).

Volume 9 started during the end of 2017 and ended at the beginning of 2018.

Mal used to separate them, but merged them together quite a while ago, that's probably why you're confused about it.

You can watch the anime, I highly recommend it because it's a perfect adaptation which did the manga justice and maybe even surpassed it.

You can then read vol 9 if you want or not that is up to you.
You can also choose which ending you like more, everyone has a different preference.

Enjoy it of you haven't already and I hope I could help.