October Novel Prep Challenge

I think the answer to that is in the story you're telling. Some novels only need a couple characters and they hold all the action between them. Others require major and minor charterers, only some of whom need an arc.

I'm thinking about Run, by Ann Patchett. Entire book takes 24 hours. There is a family involved: two parents and two boys who meet a girl and her mother because of an accident. Minor character as a nurse or University employee? Not needed. The father, both boys, the girl have arcs. Both mothers add layers to the tale. It's literary so if you have a commercial market in mind it'll be different.

Don't think you need a specific number. Tell the story and allow it to be a guide. Sometimes a second draft can weave a character through different positions- George RR Martin did this for the show GoT where in the books he could have a larger cast. (I guess Hollywood didn't want to kill that many extras every week.)