CREATE TABLE table_18155 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" TEXT, "Results" TEXT, "Candidates" TEXT)

How many elections have resulted in retired democratic hold?
SELECT COUNT("Party") FROM table_18155 WHERE "Results" = 'Retired Democratic hold'
SELECT COUNT("Party") FROM "table_18155" WHERE "Results" = 'Retired Democratic hold'