How many different lead margins were stated in the poll administered on September 11, 2008?
CREATE TABLE "opinion_polling_for_the_united_states_gu" (
  "poll_source" text,
  "dates_administered" text,
  "democrat_jay_nixon" text,
  "republican_kenny_hulshof" text,
  "lead_margin" text
);
SELECT COUNT("lead_margin") FROM "opinion_polling_for_the_united_states_gu" WHERE "dates_administered"='September 11, 2008';