CREATE TABLE table_18258 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" FLOAT, "Result" TEXT, "Candidates" TEXT)

how many candidates with result being retired to run for u. s. senate republican hold
SELECT COUNT("Candidates") FROM table_18258 WHERE "Result" = 'Retired to run for U. S. Senate Republican hold'
SELECT COUNT("Candidates") FROM "table_18258" WHERE "Result" = 'Retired to run for U. S. Senate Republican hold'