How many constructors are listed for the XVI BRDC international trophy race
CREATE TABLE "non_championship_race_results" (
  "num" real,
  "race_name" text,
  "circuit" text,
  "date" text,
  "winning_driver" text,
  "constructor" text,
  "report" text
);
SELECT COUNT("constructor") FROM "non_championship_race_results" WHERE "race_name"='XVI BRDC International Trophy';