On July 17, 2008, what was the total number of lead maragin? 
CREATE TABLE "opinion_polling_for_the_united_states_se" (
  "poll_source" text,
  "dates_administered" text,
  "democrat_mark_begich" text,
  "republican_ted_stevens" text,
  "lead_maragin" real
);
SELECT COUNT("lead_maragin") FROM "opinion_polling_for_the_united_states_se" WHERE "dates_administered"='July 17, 2008';