Eddie Brimer wrote:

there doesn't have to be less than 10 of anything to be rare. gimme a
break. this is a big world.

I submit that in our specialized field, ten is a good number.
The point is that a "rare" set should be very difficult to acquire;
that it should take a large portion of effort and time to find one,
so that you can expect only a couple of such sets
to come to your collection.
This separates the most prized sets from the "Uncommon,"
which also require time and effort to find,
but not to the degree a "Rare" set requires.
An "Uncommon" set should be a source of pride and satisfaction.
A "Rare" set should feel more like a blessing.

Kindly,
David S.