Also, if polenta can be made with frozen corn, how much should be used?

Frozen would work as well, I suspect. I would follow the guidelines above. Frzn corn tends to have a lot of water clinging to it – give it a good rinse in a sieve and dry in a tea towel before using. At this time of year frzn corn is probably a better choice than what one can get on a cob.

I have successfully substitute fresh corn kernels with corn kernels from a can. Choose canned corn that has nothing added to it (an not the creamy kind). Drain well and pat fry. This recipe calls for 1 1/2cups
You can substitute canned corn for the fresh kernels. Choose ones that have nothing added and not creamy. Rinse the corn well and pat dry. This recipe calls for 20oz of corn kernels which is about 3.5-4 cups of kernels