user: Question: How many different team captains does the club Sliven 2000 have?
Columns: ['Club', 'Location', 'Manager', 'Stadium', 'Capacity', 'Team captain']
Types: ['text', 'text', 'text', 'text', 'real', 'text']
Sample row: ['Beroe', 'Stara Zagora', 'Ilian Iliev Category:Articles with hCards', 'Beroe', 17800.0, 'Slavi Zhekov']
SQL:

assistant: SELECT COUNT("Team captain") FROM "Table" WHERE "Club" = 'Sliven 2000';