Expanding a little further… just bear in mind, all people are interesting. Some might be dreadful, but they’ll still have plenty of complexity to them. So this is something to think about even when writing minor characters, who might have very brief and little screentime… these people still have their own stories, and are still people, and if you can bring them just a little spark of characterization, your story will feel that much more real. A big cast will generally entail some people being in the background more, but if you think of them as having their own lives, it helps your world feel full.
Big casts can add a lot of appeal, too… there’s more range for variety, readers have more chances to find someone they particularly connect with, and it can lend more room for dynamics in the interactions and plots. Just look at how many conversations, on this forum and elsewhere, deal with large bodies of characters and the ways the readers react to different ones.
For distinctness, I’d also say, think about people whom you know… and maybe think about people you encounter on the forum. After all, this forum is text-based, so it’s a prime example of how to get to know people on a word-by-word basis. Consider what helps you remember who’s who—what sorts of traits stand out?
People can be fairly similar too, and have it still make sense for a story—you just need to be sure you bring out what makes them unique, as well. I mean, you may want to write about family, or friends, or people in a relationship, who are likely to have things in common. I guess, if I may use an example, @ParrotWatcher and I are fairly similar in a lot of ways, but we’d still be noticeably distinct, in a way where, if we were characters, it wouldn’t be redundant so much as complementary… and readers could keep us apart through things like, well, he’s British and I’m American, he’s more introverted than I am (not that I’m an extravert ), he’s more into superhero stuff, he dislikes bananas, etc… you can do exactly the same sort of thing with fictional characters
And I’d also say, maybe don’t introduce too many characters all at once—it’s a bit much to process. You can space them out a little