user: Question: how many premieres had the episode that had 12.68 millions of north american viewers? 
Columns: ['No. in series', 'No. in season', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code', 'U.S. viewers (millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'real', 'text']
Sample row: [350.0, 1.0, '"Red Ball"', 'Matthew Penn', 'David Wilcox', 'September21,2005', 16001.0, '13.03']
SQL:

assistant: SELECT COUNT("Original air date") FROM "Table" WHERE "U.S. viewers (millions)" = '12.68';