CREATE TABLE table_18158 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" TEXT, "Results" TEXT, "Candidates" TEXT)

How many incumbents were first elected in 1984?
SELECT COUNT("Incumbent") FROM table_18158 WHERE "First elected" = '1984'
SELECT COUNT("Incumbent") FROM "table_18158" WHERE "First elected" = '1984'