What was the series# with the original airdate of october 23, 1967?
CREATE TABLE "table1_21146729_6" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "director" text,
  "writer_s" text,
  "original_airdate" text
);
SELECT MAX("series_num") FROM "table1_21146729_6" WHERE "original_airdate"='October 23, 1967';