user: Question: Name the number of reason for change on May 11, 1939
Columns: ['District', 'Vacator', 'Reason for change', 'Successor', 'Date successor seated']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Maryland 5th', 'Vacant', 'Rep. Stephen W. Gambrill died in previous Congress', 'Lansdale Sasscer (D)', 'January 12, 1943']
SQL:

assistant: SELECT COUNT("Reason for change") FROM "Table" WHERE "Date successor seated" = 'May 11, 1939';