A lot of people now are frustrated with contemporary series being much shorter than they used to be. Down from 20-24 episodes a season in the 90s and 00s to about 10-12 in the 10s and 6-10 now (varies somewhat by streaming service and country). People are also frustrated with the big gaps between seasons (The Last of Us, HOTD, Stranger Things) which I do completely agree with and think needs to change.
My ideal is about 10-12. I personally really dislike 20-24 episode long series, and dislike the “monster of the week” formula that many older series used to have. At least for dramatic shows. For sitcoms and adult animation it makes more sense due to much shorter episodes and a more flippant and less high-stakes style of writing (although I don’t gravitate so-much to that type of show in the first place).
For me a lot depends on how well they can sustain the writing over the season. Some of the longer running seasons, hell, even some shorter seasons had episodes that are clearly filler because they had a fixed quota of episodes but not quite enough script and/or budget to cover all of them to the same standard.
I think most people will be familiar with the Breaking Bad episode Fly, which is the worst rated by some margin on the entire show. It had its budget stolen for the episodes around it, and frankly it sucks. It would have been better if they just went one episode less that season. This is why I am in favor of the show only having enough episodes that it can actually sustain with great content, and may be that number varies season to season.
Monster of the week shows like Buffy that have a season long big story arc and usually single shot weekly stories it is even more important not to over do it with poor quality filler. and over stretch the big story arc to the point it becomes boring.