Back with newsgroups the general rule was to go from general to specific. You start with a general discussion group and when discussions about video games get annoying you create a games group. If then there are too many Baldur’s Gate discussions you create BG. If they are dominated by Baldur’s Gate 3 you create a Baldur’s Gate 3 group. If everyone is fawning over Withers you create a Withers group which of course will be flooded with discussion about the Withers’ tits mod, which shall get its own group.
Meaning you should create a group when demand is there and not the other way around.
I think something you’re missing is that “we create communities as needed” has an inverse as well. “We delete communities as needed”. Sometimes you create the general topic and it’s so general that all of the niches overtake it. When that happens and the general just isn’t needed, you prune it. No community has to exist forever and sometimes it’s only purpose will be as a reference point to others. And sometimes even that isn’t needed anymore and it vanishes too.
It’s a constantly changing, dynamic system. The point is that it should cater to what’s needed/being used at the moment.