CREATE TABLE table_203_491 (id DECIMAL, "year" DECIMAL, "title" TEXT, "us hot 100" DECIMAL, "us modern rock" DECIMAL, "us mainstream rock" DECIMAL, "album" TEXT)

how many singles reached the us mainstream rock chart ?
SELECT COUNT("title") FROM table_203_491 WHERE NOT "us mainstream rock" IS NULL
SELECT COUNT("title") FROM "table_203_491" WHERE NOT "us mainstream rock" IS NULL