Help me find this book!!

There’s this book I read a long time ago that I just cannot remember the name or author of.

The title was a number, it had a green background.

It was about this guy who comes across this group of people who purposefully put themselves through chemo therapy even though they’re not sick. It’s like a weird cult. He ends up joining them and they all go by numbers for their names, their numbers are for when they joined the group and what number member they are. I want to see what else this author has written as this one was a really good thriller!! Pls help 😭😭