The in-game documentation for this setting is not clear enough and it always causes confusion. If it’s set to a number greater than 0, it indicates how many players from a clan have to be online for that clan to be eligible to be Purged. For example, if you set it to 2, and there’s only one player from clan Foo online, then clan Foo won’t be Purged.
However, if it’s set to 0, there needs to be only 1 online player on the whole server, regardless of his/her clan, for a Purge to happen. That means that offline Purges will happen for clans that have no players online, as long as there’s someone on the server.