What is the capacity for a facility opened in 1956?
CREATE TABLE "current_prisons" (
  "facility" text,
  "location" text,
  "year_opened" text,
  "major_facility" text,
  "population_gender" text,
  "capacity" real,
  "custody_level_s" text
);
SELECT COUNT("capacity") FROM "current_prisons" WHERE "year_opened"='1956';