A good principle for representation in stories, one I've heard and want to pass on, is that the ratio of races/identities among characters should be the same as in that society at large. If the story digs into inequities, then of course a more exact realism can make sense or become necessary (and the digging will give balance). But, for example, it's at least slightly noxious or untoward for a story about programmers in America to involve only white men. That is, don't break down the current subculture demographically, as it could easily reflect prejudice in hiring. Represent the wider cultural one. Use the real background numbers. Roll some virtual dice. Or hand-pick your cast, but count. Only deviate from the rule when there's a specific reason. Fiction allows and asks us to draw and change with awareness. And this isn't a strict rule, that is I'm not proposing eviscerations where it's broken, but it is a good guideline. I can't think of a better one.

By the same token, half of characters should be women, or even slightly more than half. I don't care as much about religion so I'm not emphasizing it as much, but the argument is the same, and it might make for a better story. Trying to match the wider demographic background actually, to me, suggests more creative possibilities than trying to profile the audience and cater to them with what they tend to like. The latter easily slips into pandering and cliche. The former is a source of ideas you wouldn't think of. Imagine that.

There is an argument that varying race/gender/etc can make the story seem to be about that when it isn't. But I'd counter that this mainly seems true because the rule isn't followed enough yet, and so when it is, that stands out. If it were usually followed, it wouldn't stand out, and that might result in an actually more effective spin on color-blindness. In other words, with representative mixing we'll stop thinking everything is about race and politics, because representation has reached parity and we've gotten used to seeing what's actually there.