How many hamsters is too many when my cat is hungry?

When I go to work in the morning, I usually just put my cat in his travel case, drop in a hamster, close the door, and come back in the afternoon.

If the cat has eaten the hamster, I’ll let him out. If not, I leave him in there for a while longer.

Now that my cat is getting to be an adult, how many hamsters a week should I be feeding him?