user: Question: Name the total number of reason for change for Not filled this congress
Columns: ['District', 'Vacator', 'Reason for change', 'Successor', 'Date successor seated']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['South Carolina 6th', 'Vacant', 'Rep. Warren R. Davis died during previous congress', 'Waddy Thompson, Jr. ( AJ )', 'Seated September 10, 1835']
SQL:

assistant: SELECT COUNT("Reason for change") FROM "Table" WHERE "Date successor seated" = 'Not filled this congress';