I noticed that a post has been flagged, from the Community user I guess, because it had more than 20 comments.
What is the purpose of that flag, and what should be done in such cases?
High comment frequency can be an indication of a number of factors.
It's simply a warning to moderators that "Something may be amiss". Like most of Community user's boilerplate flags, it's only going by bare metrics so it's not a guaranteed actionable element. As noted, none of the resolutions are much that a non-♦ can do much about other than add additional flags.
That can be useful, though. You can note in a flag about the particular instance, if you think it is something that should be investigated. Or, you can say Community's flag is invalid, if there's nothing important.