There are two basic kinds of shells, hard and soft. The hard are mostly calcium, like oyster or clam shells. The soft generally translucent parts are mostly chitin, a protein, not calcium. Think shrimp shells but not the claws. The chickens can eat chitin but it doesn’t digest all that well. They’ll get some benefit from it but not a lot. Still mine eat it and really like it, generally shrimp shells. They can pick the softer shrimp shells apart. Don’t expect them to do that with the hard calcium parts.
On a lobster or crab you will have both and some parts are a mixture of both. The harder it is the higher the calcium content. The softer it is the higher the chitin content.
I doubt you can feed them too much of either. If you crush the hard parts it’s basically like oyster shell, they will self-regulate how much they eat. I’m less certain on the chitin but would think they might self-regulate there also.
I don’t think I’d put that directly in my garden. The hard calcium just won’t break down for a long time and a broken shell can be kind of sharp. I would not want to be digging in that with bare hands.
I don’t know how long it would take for the chitin to break down. You might be better off putting it in your compost pile than directly in the garden. Every gardener with chickens has to have a compost pile of some sort. You can turn chicken poop, garden and kitchen wastes, and yard waste like clippings and leaves into black gold. There is nothing better for a garden than compost.
