CREATE TABLE table_30030 ("Series #" FLOAT, "Season #" TEXT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "U.S. viewers  ( millions ) " TEXT)

How many series numbers are there when there were 15.8 u.s. viewers (millions)?
SELECT COUNT("Series #") FROM table_30030 WHERE "U.S. viewers (millions)" = '15.8'
SELECT COUNT("Series #") FROM "table_30030" WHERE "U.S. viewers (millions)" = '15.8'