Besides the default/obvious channels, we generally seem to have a channel per "team", a channel per "division" (group of teams), and then some channels for "division collaborations" e.g. data science + engineering. Then we also often set up a channel for big new features/projects and invite stakeholders to them.

EDIT: I guess the last type could be what you mean by "topic" - we are a startup so perhaps the number of feature/project type channels will balloon out over time, but doubt we would end up with "zillions" haha. And loads of them would be old and essentially archived.

A lot of the channels become somewhat bloated over time, with people joining because they are interested in keeping up with stuff in a broad context, but have no need to click into each thread and read over everything.

I would choose threaded discussions over PM groups or short-lived channels any day tbh.