How many figures are given for the New Democratic for the polling range May 11–31, 2010?
CREATE TABLE "opinion_polls" (
  "polling_firm" text,
  "date_of_polling" text,
  "link" text,
  "progressive_conservative" real,
  "liberal" real,
  "new_democratic" real
);
SELECT COUNT("new_democratic") FROM "opinion_polls" WHERE "date_of_polling"='May 11–31, 2010';