How many 2001 censuses are there on number 13?
CREATE TABLE "references" (
  "num" real,
  "city" text,
  "1981_census" real,
  "1991_census" real,
  "2001_census" real,
  "2010_est" real,
  "region" text
);
SELECT COUNT("2001_census") FROM "references" WHERE "num"=13;