There are two meanings for "corn". 1 chiefly Brit. the chief cereal crop of a district, especially (in England) wheat or (in Scotland) oats. 2 N. Amer. & Austral./NZ maize, the ears of which are corn on the cob. Corn is uncountable in both cases. You would ask for "two cobs".
The ears of all corn are the seed-bearing heads.