How many champions are shown for the runner-up of stefan edberg?
CREATE TABLE "table1_21584646_10" (
  "week_of" text,
  "tournament" text,
  "champion" text,
  "runner_up" text,
  "semifinalists" text,
  "quarterfinalists" text
);
SELECT COUNT("champion") FROM "table1_21584646_10" WHERE "runner_up"='Stefan Edberg';