How many values of attendance occur when ave. on previous season is +645?
CREATE TABLE "average_crowd_sizes" (
  "season" text,
  "ave_attendance" real,
  "ave_on_prev_season" text,
  "competition" text,
  "occupier" text
);
SELECT COUNT("ave_attendance") FROM "average_crowd_sizes" WHERE "ave_on_prev_season"='+645';