Name the number of constr with the first win at the 1978 brazilian grand prix
CREATE TABLE "table1_12707313_2" (
  "pos" real,
  "manufacturer" text,
  "seasons" text,
  "starts" real,
  "wins" real,
  "sole_supplier" real,
  "first_win" text,
  "last_win" text,
  "drivers" real,
  "constr" real
);
SELECT COUNT("constr") FROM "table1_12707313_2" WHERE "first_win"='1978 Brazilian Grand Prix';