I can find details of how much protein per body of fluid (e.g. 1 US cup) but to be honest we probably all make stock to a different concentration, so that makes that metric useless.
When I prepare a large chicken (2kg), I remove the breasts , wings and legs (and the oysters on the back) so I'm left with just the main carcass which I then simmer for ages. How much protein is extracted please?