R-1: Greater than 5,000. R-2: Between 2,000 and 5,000. R-3: Between 500 and 2,000.
R-4: Between 200 and 500. R-5: Between 76 and 200. R-6: Between 21 and 75. R-7: Between 11 and 20. R-8: Between 5 and 10.R-9: Between 2 and 4. R-10: Unique (one known example).