user: Question: What is the number of vacators when the successor was William H. Wells ( F )?
Columns: ['State (class)', 'Vacator', 'Reason for change', 'Successor', 'Date of successors formal installation']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Tennessee (1)', 'Vacant', 'Tennessee failed to elect a Senator on time', 'William Cocke ( DR )', 'Appointed May 15, 1797']
SQL:

assistant: SELECT COUNT("Vacator") FROM "Table" WHERE "Successor" = 'William H. Wells ( F )';