List the number of bbc rankings with cable rankings of 6.
CREATE TABLE "table1_24399615_10" (
  "episode_no" real,
  "airdate" text,
  "viewers" text,
  "bbc_three_weekly_ranking" text,
  "cable_rank" text
);
SELECT "bbc_three_weekly_ranking" FROM "table1_24399615_10" WHERE "cable_rank"='6';