How many different riders are there that won riding Omr Tsunami?
CREATE TABLE "world_championship_winners" (
  "year" real,
  "location" text,
  "distance_miles" real,
  "rider_names" text,
  "horse_name" text,
  "best_conditioned_horse" text
);
SELECT COUNT("rider_names") FROM "world_championship_winners" WHERE "horse_name"='OMR Tsunami';