user: Question: How many different original air dates does the episode with series number 13 have?
Columns: ['Series No.', 'Season No.', 'Title', 'Directed by', 'Written by', 'Original air date', 'U.K. viewers (million)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: [13.0, 1.0, '"The Fashion Show"', 'Ben Palmer', 'Damon Beesley & Iain Morris', '13September2010', '2.25']
SQL:

assistant: SELECT COUNT("Original air date") FROM "Table" WHERE "Series No." = 13.0;