What % of the corn crop is sold on the cob?
For field, or dent corn, which is used for non-human use, feedstock, ethanol, plastics, etc. practically none.
The combines, or field harvesters, separate the kernels from the cob right in the field.
Sweet corn, the kind you buy in the store, to be candid, I don't know. Overall however, sweet corn is only a small fraction of total corn production.