It usually is not the particular number of the CA 19-9 that is important, but rather the trend of the numbers. If the numbers have been trending up, and your father is on chemo, it generally means the chemo is no longer working and should be changed to a new chemo protocol. Have you discussed this with your father's oncologist? It is important not to go for longer than absolutely necessary without chemo.
Best wishes for you and your father.
Joan L