It used to be 26 episodes per season, with each episode airing twice during the year. It was a nice, simple way of filling the broadcast schedule. That shifted to 24 episodes at some point. There was a bigger shift, I think in the early 2000s, where they started having separate shows for the summer, and seasons started getting much shorter, sometimes more like 13 episodes. Now streaming services will put out 6-7 episode seasons; only a quarter of what a season used to be.
The good part of this is that you no longer get filler episodes. I remember watching shows like Stargate SG-1, and there were inevitably a few junk episodes, like a clip show that has some excuse to edit together a bunch of clips of previous episodes, or some episode that really didn't do much because they clearly spent all their budget already. I don't miss those. But with only 6 episodes, it's down to the same run-time as a miniseries, and things sometimes feel rushed.
For shows that are telling a story over the course of a season, the shorter episodes sometimes work well, but for more episodic shows (like Doctor Who), it just feels like you're getting shorted (because you are).
For many shows, the driving force is the quality of the writing and acting. Would the studios do better to spend less on the production and get more episodes for the same money? Good stories outweigh good effects.