user: Question: How many reasons were given when A. Willis Robertson (D) resigned?
Columns: ['State (class)', 'Vacator', 'Reason for change', 'Successor', 'Date of successors formal installation']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['South Carolina (3)', 'Olin D. Johnston (D)', 'Died April 18, 1965', 'Donald S. Russell (D)', 'April 22, 1965']
SQL:

assistant: SELECT COUNT("Reason for change") FROM "Table" WHERE "Vacator" = 'A. Willis Robertson (D)';