I think limiting the number of post tags is not a good idea. Of course it's probably for the better if a community can't have 200 different tags for "active use", but for a community with a broad topic (which is common at this time in Lemmy), tags would be useful for the kind or the topic of the post one makes.
With kind I mean "question", "meme", "news" and "sale" among others, or also if a kind of post is only allowed on a certain day of the week, a useful rule could be the mandatory use of the corresponding tag on it, so those whom it bothers can filter it out, and the posts are also searchable based on that for those interested. Adding to this, as time goes on and the day-limited topics change, you (as a community moderator) probably don't want to delete their tags to keep the posts of it searchable, just as it was originally. Archiving tags (like Gitea will do apparently in the next major version) would be a useful feature for this case, to keep them for searchability, but also keep them out of the way of actively used tags for when you tag your new post. It is basically a boolean variable for each tag, and sorting of tags in menus based on its state.
And with topic, it would be a similar use case but for a recently very hot topic, but to be honest I can't see this one being used by posters, as I imagine these would be very volatile, and as I understand posts can only have tags that were declared by mods for the community.

If we have a limited number of community specific tag, at least make the limit configurable by the server admin.