CREATE TABLE table_204_511 (id DECIMAL, "number" DECIMAL, "driver" TEXT, "entrant" TEXT, "chassis" TEXT, "engine" TEXT, "tyre" TEXT)

how many racers had an offenhauser engine ?
SELECT COUNT("driver") FROM table_204_511 WHERE "engine" = 'offenhauser'
SELECT COUNT("driver") FROM "table_204_511" WHERE "engine" = 'offenhauser'