user: Question: How many successors were seated in the New York 20th district?
Columns: ['District', 'Vacator', 'Reason for change', 'Successor', 'Date successor seated']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Pennsylvania 18th', 'Robert J. Corbett (R)', 'Died April 25, 1971', 'H. John Heinz III (R)', 'November 2, 1971']
SQL:

assistant: SELECT COUNT("Successor") FROM "Table" WHERE "District" = 'New York 20th';