user: Question: How many districts have John C. Calhoun as the incumbent?
Columns: ['District', 'Incumbent', 'Party', 'First elected', 'Result', 'Candidates']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['South Carolina 2', 'William Lowndes', 'Democratic-Republican', '1810', 'Re-elected', 'William Lowndes (DR) 100%']
SQL:

assistant: SELECT COUNT("District") FROM "Table" WHERE "Incumbent" = 'John C. Calhoun';