How many share figures are there for the episode that aired on October 17, 2007?
CREATE TABLE "u_s" (
  "num" real,
  "episode" text,
  "air_date" text,
  "timeslot_est" text,
  "rating" text,
  "share" real,
  "18_49_rating_share" text,
  "viewers_m" text,
  "weekly_rank_num" real
);
SELECT COUNT("share") FROM "u_s" WHERE "air_date"='October 17, 2007';