So sorry to hear about your father I am sure this is so difficult to live through for you.
The scale for this test is 9.5-49 micrograms per deciliter (mcg/dL) I am sure there is not a specific number for coma as it likely varies between patients.
The symptoms he experiencing is called hepatic encephalopathy which is related to high ammonia blood levels.
Does he have any other symptoms of cirrhosis like ascities, jaundice or muscle waisting?
I am assuming your father has advanced decompensated liver cirrhosis. Is he eligible for a liver transplant?
Unfortunately the only thing that can change his prognosis at this point if he does have decompensated cirrhosis is a liver transplant.
Have you spoken to his doctors about his prognosis and what his options are? They really will be best to advise you about what to expect.
I am assuming he is in the care of a liver transplant center unless he is in hospice for palliative care.
My heart goes out to you father you and his family in this difficult time.