how many new managers replaced  manager(s) who resigned? 
CREATE TABLE "managerial_head_coach_changes" (
  "team" text,
  "outgoing_manager" text,
  "manner_of_departure" text,
  "date_of_vacancy" text,
  "incoming_manager" text,
  "date_of_appointment" text
);
SELECT COUNT("incoming_manager") FROM "managerial_head_coach_changes" WHERE "manner_of_departure"='Resigned';