Nigeria had the fastest time once.
CREATE TABLE "junior_under_20_women" (
  "rank" real,
  "fastest_time_s" text,
  "wind_m_s" text,
  "athlete" text,
  "nation" text,
  "date" text,
  "location" text
);
SELECT COUNT("fastest_time_s") FROM "junior_under_20_women" WHERE "nation"='Nigeria';