When 30% is scott mcadams (d) percentage how many poll sources are there?
CREATE TABLE "polling" (
  "poll_source" text,
  "dates_administered" text,
  "joe_miller_r" text,
  "lisa_murkowski_r_write_in_candidate" text,
  "scott_mc_adams_d" text
);
SELECT COUNT("poll_source") FROM "polling" WHERE "scott_mc_adams_d"='30%';