CREATE TABLE table_17975 ("Value" TEXT, "Diameter" TEXT, "Thickness" TEXT, "Mass" TEXT, "Composition" TEXT, "Edge" TEXT, "Obverse" TEXT, "Reverse" TEXT, "first minting" FLOAT, "issue" TEXT, "withdrawal" TEXT, "lapse" TEXT)

How many coins have a diameter of 23mm?
SELECT COUNT("Reverse") FROM table_17975 WHERE "Diameter" = '23mm'
SELECT COUNT("Reverse") FROM "table_17975" WHERE "Diameter" = '23mm'