CREATE TABLE table_18591 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" FLOAT, "Result" TEXT, "Candidates" TEXT)

How many districts have an incumbent first elected in 1940?
SELECT COUNT("District") FROM table_18591 WHERE "First elected" = '1940'
SELECT COUNT("District") FROM "table_18591" WHERE "First elected" = '1940'