Something that can make or break an entire anime is how quickly a series introduces its characters. Even long-running anime have a problem of suddenly providing a group of characters and not elaborating until later, to the point where it can put some viewers off to suddenly get a chunk of cast they don’t want to be acquainted with and drop the anime in question.
Well, first of all, you could just have a smaller cast. For anime based around teams, this is pretty hard unless the series runs long enough to elaborate on anything and everything about every character, or deliberately devote resources to each team in turn (see, for instance, how Hypnosis Mic splits its manga up so that it only focuses on two divisions per manga, or the past group in a certain manga storyline).
You could increase focus on certain characters and decrease focus on others. This is generally what long-runners do – put certain groups in focus at a time as the plot demands it, such as how Detective Conan has different prefectural police teams and other allies in certain places (e.g. you almost only see Heiji and Kazuha in Osaka-related cases). However, fine-tuning such a thing is difficult, especially when certain members of the cast prove more popular than others.
Furthermore, instead of extending the storyline so that you wrap up every character’s loose ends, you manage the cast so that you don’t have to focus on so many characters. Lately, Shonen Jump adaptions have been running shorter than they used to, not only because people move on more easily in a media-saturated landscape like ours, but because it’s easier to manage the cast if it’s significantly smaller. See, for instance, how Astra manages to stay interesting with a main cast of 9 (later 10) people.
What does this mean for writers, both inside and outside anime/manga? It means they have to be careful moving forward, because while it’s easy to let your cast expand indefinitely (notably, Bleach‘s mangaka was known for introducing new characters to solve his writer’s block), the shorter the series, the harder it is to rein in your characters without adequately wrapping up their conflict.
This is clearly not an issue related just to anime and manga – it’s just particularly noticeable due to those media’s tendency to have cast herds (as they are known on TV Tropes). So, why do people need to introduce all their characters at once and what can be done about it?
I always like how iDOLM@STER did it… They introduced the whole cast in the first episode, but then shifted to a more “individual character focus episode” structure dealing with each character in turn. But even so, you learned about the not focus character through their actions and interactions in their not focus episodes.
Basically, they built everything up layer-by-methodical-layer in a very deliberate manner. Pretty much all large cast anime I enjoy work something like this. All too many times, shows try and introduce too many characters _and their characteristics and quirks_ all at once. Then, the characters doesn’t matter until suddenly the thirty second introduction becomes Really Important in episode 7… That’s just lazy writing.
Basically, you can’t shove a main character into the background after introduction and forget about them. You have to keep them and their important characteristics and quirks in mind more-or-less constantly. Certainly not every episode, or at least not big scenes every episode, but in an ongoing manner.