How many episodes have 18.73 million viewers?
CREATE TABLE "table1_27481781_2" (
  "no_in_series" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "prod_no" real,
  "viewers_millions" text
);
SELECT COUNT("no_in_series") FROM "table1_27481781_2" WHERE "viewers_millions"='18.73';