CREATE TABLE table_28423 ("No." FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "U.S. viewers  ( million ) " TEXT, "Rank  ( week ) " FLOAT, "Original air date" TEXT, "Production code" TEXT)

How many air dates does the episode with 15.50 million viewers have?
SELECT COUNT("Original air date") FROM table_28423 WHERE "U.S. viewers (million)" = '15.50'
SELECT COUNT("Original air date") FROM "table_28423" WHERE "U.S. viewers (million)" = '15.50'