SELECT COUNT("Lead Maragin") FROM table_21046 WHERE "Dates administered" = 'July 17, 2008'
On July 17, 2008, what was the total number of lead maragin?
CREATE TABLE table_21046 (
    "Poll Source" text,
    "Dates administered" text,
    "Democrat: Mark Begich" text,
    "Republican: Ted Stevens" text,
    "Lead Maragin" real
)