1）a corn on-the-cob/a corn on the cob
two corn on the cobs.
The corns on cob are pretty good.

2) a corn cob /two cobs of corn

Hopefully my most recent visit will give you an idea of how much food you'll get for $80 (including tip and tax): 4 sodas, 1 order of sweet potato fries, 1 order of cajun fries, 1 catfish and cajun fries, a dozen raw oysters, 3 pounds of shrimp, 2 cobs of corn, 1 side of potatoes, 2 sides of 6 sausage pieces, and 6 dipping sauces. Not bad, not bad!

以上均来自网上，个人以为,下面的用法较为实用一点，毕竟在老美的餐厅就这样标价计数
a cob of corn
two cobs of corn

其他用法，对复数和冠词等表示似无规律。语法学家可以研究并规范之。