How many doubles champions were there for the Serbia f6 Futures tournament?
CREATE TABLE "events" (
  "tournament" text,
  "date" text,
  "city" text,
  "country" text,
  "surface" text,
  "singles_champions" text,
  "doubles_champions" text
);
SELECT COUNT("doubles_champions") FROM "events" WHERE "tournament"='Serbia F6 Futures';