What is the number of participating parties when registered voters totaled 643629?
CREATE TABLE "congress" (
  "electoral_district" text,
  "registered_voters" real,
  "seats_in_congress" real,
  "candidates_per_party" real,
  "participating_parties" real,
  "total_candidates" real
);
SELECT COUNT("participating_parties") FROM "congress" WHERE "registered_voters"=643629;