I have oyster shell for them. Yes, I will feed the whole corn in moderation. Becky (The youtube channel person) gives her hens 1 cup of whole corn in the morning and one if the evening. I think she as about 11 hens.
Oyster shell is different than granite grit.
The grit is actual granite in small enough bits they can eat it. It stays in the gizzard and helps grind the food for further digestion.
Oyster shell is a calcium source to aid in keeping shells strong.
Feed stores also carry grit.
That is a LOT of corn for just 11 birds.