When don inglis and ralph smart are the writers how many episode numbers are there?
CREATE TABLE "table1_25046766_1" (
  "episode_no" text,
  "prod_no" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_uk_airdate" text
);
SELECT COUNT("episode_no") FROM "table1_25046766_1" WHERE "written_by"='Don Inglis and Ralph Smart';