I wouldn't go higher than 12 episodes for modern Trek . Actors like having flexibility these days to do other things . I also don't think the modern showrunners have enough creative juices to pull it off .
Having said that, more episodes for SNW is possible due to it's episodic format. But Anson Mount probably wouldn't want more. Which parallels Paramount profit margin budget anyway at limiting it to 10.
I'm pretty sure most of the SNW cast would want more episodes, short seasons are not a benefit for most actors and writers, on the contrary, it makes it very hard for them to make a living. That was a major reason for the strikes last year.
I specifically remember Michelle Hurd and Katee Sackhoff talking about this, actors and writers get paid per episode, if you only do 10 episodes per season instead of 20+ that's obviously significantly less money earned from the start and often you're not even doing one season per year, the breaks between seasons and hiatuses between shooting become extremely long and while actors can in theory do other work during the hiatus in practice most studios will not hire the actors at least not for major parts because while they are on hold for another show they can be called back to that show at any point making them unavailable for the other studio and forcing them to drop out.
So after a season wraps an actor might sit on their ass for sometimes 12 months, not knowing if they'll even be back for the show (because on hold just means the studio
can call them back for another season not that they have to) and finding it very hard to get other jobs during that year, a bit part that's shot over a few day is possible but forget being the lead in a movie that shoots for 8 weeks, no production would take that risk.
So you earn less money upfront, you don't really have many opportunities to take other jobs during the hiatus and the residuals are also mostly non existent now. Will Wheaton said he was able to live off his TNG residuals for years while he wasn't working a lot, this is no longer happening with modern shows, for a streaming show you get paid a lump sum up front and that's pretty much it. One actress who was on dozens of episodes of Orange is the new Black, a major hit for Netflix and a long running show and she got about $100 in residuals, that's a joke.
Netflix and the unsustainable business model they introduced is literally destroying the industry, many actors and writers are no longer getting paid enough to make a living, the home video market has pretty much collapsed which is the reason why mid budget movies have disappeared from theaters because they relied on home video as a second revenue stream to make them profitable and all we're left with are blockbusters and franchise movies because those are the only ones people still get out of the house for, try to release a mid budget movie and most of the people will shrug and say "I wait until it's streaming". TV budgets for "prestige tv" have exploded but somehow large parts of the audience also want them for basically free, "Weekly release? Screw that, I'm not paying for three months just to watch a season of show x, I'll subscribe for a month after the finale and binge it!", a very expensive season but people are only willing to invest 5-10 bucks to watch it, they want premium entertainment at bargain prices.
The point I was trying to make is that if DS9 was designed and written with the same quality of writing that went into SNW, 15 episode seasons would still make an excellent show.
15 episodes a season of DS9 would probably mean we'll never get Garak or Nog focused episodes, you don't waste time on recurring characters when you only have slightly more episodes than main cast members and a bigger story to tell, many recurring characters might not even exist. Rom and Nog might have been folded into the same character for example, a younger brother for Quark old enough to work at the bar and be an engineer but young enough to be friends with Jake (but forget going to the academy, there probably wouldn't be time to do that arc with the reduced number of episodes).
DS9 is often praised for its large recurring cast who were all well developed and got lots of character development but most of that was only possible because they did 26 episodes per season. If you remove 11 episodes per season the show wold change significantly and much of what makes DS9 excellent probably wouldn't-ät exist.
15 sounds about right.
6 is a miniseries, while 26 encouraged filler episodes of varying quality.
How do you define filler for a franchise like Star Trek? Most of the shows were episodic to begin with, what TOS or TNG episodes are filler? They had no major arcs. Even semi serialized shows like DS9 still relied on episodes not directly contributing to the bigger story. The Visitor and Far Beyond the Stars are by most people considered to be among the show's most significant episodes but they add NOTHING to the major arcs, are they filler? Should they be tossed aside?
And then there's Discovery which premiered with a completely serialized "filler free" first season and immediately the fandom complained that there are no standalone episodes. So apparently filler is bad until it's no longer there.