CREATE TABLE table_21146729_6 (series__number INT, original_airdate VARCHAR)

What was the series# with the original airdate of october 23, 1967?
SELECT MAX(series__number) FROM table_21146729_6 WHERE original_airdate = "October 23, 1967"
SELECT MAX("series__number") FROM "table_21146729_6" WHERE "October 23, 1967" = "original_airdate"