CREATE TABLE table_204_258 (id DECIMAL, "location" TEXT, "frequency" DECIMAL, "call sign" TEXT, "network" TEXT, "call sign meaning" TEXT)

which frequency has the most stations ?
SELECT "frequency" FROM table_204_258 GROUP BY "frequency" ORDER BY COUNT(*) DESC LIMIT 1
SELECT "frequency" FROM "table_204_258" GROUP BY "frequency" ORDER BY COUNT(*) DESC NULLS LAST LIMIT 1