what of the total number of date where grand prix is portuguese grand prix
CREATE TABLE "grands_prix" (
  "round" real,
  "grand_prix" text,
  "date" text,
  "location" text,
  "pole_position" text,
  "fastest_lap" text,
  "winning_driver" text,
  "winning_constructor" text,
  "report" text
);
SELECT COUNT("date") FROM "grands_prix" WHERE "grand_prix"='Portuguese Grand Prix';