What is the number of nurses in the region with an HO : Population Ratio of 1:16,791?
CREATE TABLE "table_3_total_number_of_available_human_" (
  "region" text,
  "physician_gp_specialist" real,
  "physician_population_ratio" text,
  "health_officer" real,
  "ho_population_ratio" text,
  "all_nurses" real,
  "nurse_population_ratio" text,
  "mid_wives" text,
  "mid_wife_population_ratio" text,
  "hew" text
);
SELECT AVG("all_nurses") FROM "table_3_total_number_of_available_human_" WHERE "ho_population_ratio"='1:16,791';