Catalogues list
rarity as R, RR, and RRR for the highest
rarity, the latter where maybe only one or two are known. If you have such a RRR coin it is not evidence or proof that only one or two exist, only that one or two are recorded but dozens may exist unknown to the owners.
Many discoveries of a large or small
hoard have left collectors seeing the value tumble.
I collect Indian &
Islamic coins, the sudden popularity of which has not only lead to prices going through the roof but also to the discovery of previous uncatalogued coins, several coins listed as RR or even RRR are now known to exist in larger numbers