It really depends on the show, it’s scope and the characters. When a show adds more, important characters, time will need to be dedicated to developing those characters and playing out their story arc. Also, when a show is telling a story which encompasses a large scale, with many sub-plots, that all needs screen time. And all those sub plots and threads need time to really pay off. There is also a matter of scale for events. If a particular event has been built up, over many episodes or even seasons, it probably needs to play out over a couple episodes to give it the scale and gravitas which is expected of it.

Ultimately, the number of episodes per season is really part of the story telling decision. And that decision is going to be different for different shows. But, I’d argue that, if show writers are finding themselves turning to played out tropes like “us, but evil” or “time travel to the current time” or “musical numbers in a show which isn’t a musical” then there’s probably too many episodes that season.