How many years was the number of laps 71?
CREATE TABLE "24_hours_of_le_mans_results" (
  "year" real,
  "team" text,
  "co_drivers" text,
  "class" text,
  "laps" real,
  "pos" text,
  "class_pos" text
);
SELECT COUNT("year") FROM "24_hours_of_le_mans_results" WHERE "laps"=71;