Name the number of gn divisions for mannar
CREATE TABLE "table1_24574438_1" (
  "ds_division" text,
  "main_town" text,
  "gn_divisions" real,
  "area_km_2" real,
  "sri_lankan_tamil" real,
  "sri_lankan_moors" real,
  "sinhalese" real,
  "indian_tamil" real,
  "other" real,
  "total" real,
  "population_density_km_2" real
);
SELECT COUNT("gn_divisions") FROM "table1_24574438_1" WHERE "ds_division"='Mannar';