In Round 6, how many winning drivers were there?
CREATE TABLE "race_results" (
  "rnd" real,
  "race_name" text,
  "pole_position" text,
  "fastest_lap" text,
  "winning_driver" text,
  "winning_team" text,
  "report" text
);
SELECT COUNT("winning_driver") FROM "race_results" WHERE "rnd"=6;