If a tartan is meant to denote IDENTITY, then one user is enough, but if it is meant to signify BELONGING, then there ought to be some minimum number of users or adherents before it really makes sense. I don't know that it is necessary to police that idea, but I think it is a useful one.
I wholeheartedly agree. That would seem to satisfy both sides of the equation. It would keep tartans meaningful while giving groups a benchmark for when a tartan is justified or appropriate.

So what would the minimum reasonable group size be? 10? 100? 1000?