How many episodes have a weekly rank tba and are broadcast at 8:00 p.m.?
CREATE TABLE "u_s_standard_ratings" (
  "num" real,
  "air_date" text,
  "timeslot" text,
  "rating" text,
  "share" real,
  "18_49" text,
  "viewers" text,
  "weekly_rank" text
);
SELECT COUNT("num") FROM "u_s_standard_ratings" WHERE "weekly_rank"='TBA' AND "timeslot"='8:00 P.M.';