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I largely second @valtmy. My gut is that you often don’t have more than, say, 6-7 truly main characters over the course of a novel. Unless it’s super-long or a part of a series, you simply don’t have time to flesh all of them out. The main dividing line I’d give is that main characters are complex and three-dimensional and that side characters are simple (in what we see of them) and two-dimensional. There’s always going to be grey characters that straddle the line, but that along with valtmy’s rubric would be the basic standards I’d fall back on here.