How many times did outgoing manager Bart de Roover vacated a position?
CREATE TABLE "during_regular_season" (
  "team" text,
  "outgoing_manager" text,
  "manner_of_departure" text,
  "date_of_vacancy" text,
  "replaced_by" text,
  "date_of_appointment" text,
  "position_in_table" text
);
SELECT COUNT("date_of_vacancy") FROM "during_regular_season" WHERE "outgoing_manager"='Bart De Roover';