The label on the can or bag of dog food provides good guidelines for how much to feed your puppy, based on his weight and age. You should also consult your Vet for some guidelines on how much to feed him.
Monitoring his body weight and behavior is key. If he seems to thin and lacking energy, you may be feeding him to little or he may be sick. Take him straight to the Vet for a checkup. If you are feeding him to much he will start to put on excess fat, in which case you will need to give him more exercise and cut down on the amount of food you're feeding him.
You should feed your Chihuahua puppy four times per day until he is 6 months old. Divide your Chihuahua puppies daily ration of food into four equal meals that you will feed him throughout the day. After he is 6 months old you can feed him twice or three times per day, once in the morning and once at night usually.