This constellation with a ranking of 51 has how many percentages on record?
CREATE TABLE "external_links" (
  "rank" real,
  "family" text,
  "abbre_viation" text,
  "constellation" text,
  "area_sq_deg" text,
  "area_msr" text,
  "per_cent_age" text,
  "right_ascension_hm" text,
  "decli_nation_dm" text,
  "quad" text
);
SELECT COUNT("per_cent_age") FROM "external_links" WHERE "rank"=51;