CREATE TABLE table_18541 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" FLOAT, "Result" TEXT, "Candidates" TEXT)

What is the number of candidates for pennsylvania 21
SELECT COUNT("Candidates") FROM table_18541 WHERE "District" = 'Pennsylvania 21'
SELECT COUNT("Candidates") FROM "table_18541" WHERE "District" = 'Pennsylvania 21'