K3HVG wrote:

Seems to me that there's a bit of a disconnect in the "rare" definition.
if only 10 of any item are known to exist and one may expect to have one
or two in a lifetime of collecting, these 10 units would really be doing
a hat-dance getting around to all the collectors.

Not at all. There are only 10 of one set,
but many different kinds of sets of which 10 are known.
Therefore, you can have any number of "rare" sets,
just not of the same kind.