Name the number of countries that have the us dollar
CREATE TABLE "main_issuers" (
  "currency" text,
  "country" text,
  "generic_name_or_nickname" text,
  "rating_s_p_moodys" text,
  "negotiable_debt_at_mid_2005_us_dollar_bn_equivalent" real,
  "government_financial_liabilities_as_pct_of_gdp_end_2003_source_oecd" text,
  "issuer" text,
  "internet_site" text
);
SELECT COUNT("country") FROM "main_issuers" WHERE "currency"='US dollar';