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fkwDont know if your question means only the run of the mill sets (ie, Topps, Bowman, etc.), or any postwar cards including regionals.
There are cards that are only known as a single unique example. So they are obviously rarer than any other card.
I have one that I have yet to hear of another example....
(Im also posting this to see if anyone has or knowns of any others from this set)