user: Question: How many different writers have written the episode with series number 8?
Columns: ['No. by series', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production number', 'U.S. viewers (in millions)']
Types: ['real', 'text', 'text', 'text', 'text', 'real', 'text']
Sample row: [1.0, '"Flowers for Your Grave"', 'Rob Bowman', 'Andrew W. Marlowe', 'March9,2009', 101.0, '10.76']
SQL:

assistant: SELECT COUNT("Written by") FROM "Table" WHERE "No. by series" = 8.0;