disabled people were killed as a part of the T-4 programme, if disabled people were killed in the Holocaust, it was because they were Jewish rather than their disability.
not many, most of them were killed in the T-4 programme which was in action before the Holocaust (from which one branch of the Holocaust was developed).
How many people were involved in the holocaust? Where does personal responsibility for your actions begin? Does operating a locomotive hauling jews, pows, homosexuals, and assorted other undesirables...