How much amoxicillin for a 9lb cat?

Amoxycillin for veterinary use usually comes as a powder, which reconstitutes to 50mg of amoxycillin per 1ml of medicine. Standard veterinary dose for any cat seems to be 1ml (50mg) per dose, two doses a day.

However, no medicine should ever be administered to a cat without veterinary advice. There are many medicines which are harmful or fatal to cats, which are largely harmless to people; something added to human-grade amoxycillin which is harmless to us, may be a problem for cats because of their differing metabolism.