NS (not scarce)--More than 1,000 copies known of most cards.R1--between 250 and 1,000 copies known of most cards.
R2--between 100 and 249 copies known of most cards.
R3--between 11 and 99 copies known of most cards.
R4--between 5 and 10 copies known of most cards.
R5--5 or fewer copies known of most cards
My approach is from a slightly different angle and I have calculated/estimated/guessed how hard it would be to find a sumo menko/card if you wanted to go out and purchase one today. My scale is from an auction or purchasing point of view and basically how long it would take for one to pop up at auction if you were looking today. He is my Availability Rating scale applied to each sumo set:
Common - Menko from this set are easy to obtain, available every month or two.
Scarce - Menko from this set show up from time to time, available a few times a year.
Rare - Menko from this set are hard to come by, available only once every few years.
Extremely Rare - Menko from this set are almost impossible to find, available only a few times in a lifetime.
Here are menko from the Extremely Rare R378: 1937 Rikishi Gunbai 5 set. In almost 20 years of searching, here are the only 4 menko that have showed up while searching.