user: Question: How many air dates does the episode with 15.50 million viewers have? 
Columns: ['No.', 'Title', 'Directed by', 'Written by', 'U.S. viewers (million)', 'Rank (week)', 'Original air date', 'Production code']
Types: ['real', 'text', 'text', 'text', 'text', 'real', 'text', 'text']
Sample row: [1.0, '"First Responders"', 'Davis Guggenheim', 'David Mamet', '18.50', 10.0, 'March7,2006', '1ALF79']
SQL:

assistant: SELECT COUNT("Original air date") FROM "Table" WHERE "U.S. viewers (million)" = '15.50';