CREATE TABLE table_28915 ("District" TEXT, "Incumbent" TEXT, "Party" TEXT, "First elected" TEXT, "Result" TEXT, "Candidates" TEXT)

How many districts have John C. Calhoun as the incumbent?
SELECT COUNT("District") FROM table_28915 WHERE "Incumbent" = 'John C. Calhoun'
SELECT COUNT("District") FROM "table_28915" WHERE "Incumbent" = 'John C. Calhoun'