OK so the definition is that a human has 2 arms. What happens when they don't have two arms? Are they a new thing? Or are they still a human that we should treat with the same respect we grant to everyone?

They are still human, just missing (or have extra) arms. We should treat them with the same respect and dignity as anyone else that have 2 arms. What we don't say is we don't know how many arms humans have just because some people have more or less than 2.