CREATE TABLE table_24508 ("District" TEXT, "Vacator" TEXT, "Reason for change" TEXT, "Successor" TEXT, "Date successor seated" TEXT)

Name the number of reason for change on may 11, 1939
SELECT COUNT("Reason for change") FROM table_24508 WHERE "Date successor seated" = 'May 11, 1939'
SELECT COUNT("Reason for change") FROM "table_24508" WHERE "Date successor seated" = 'May 11, 1939'