I would say three times because chihuahua puppies are so small. Do a google search on Hypoglycemia in dogs...basically its when a dogs blood surgar levles drop and that can become deadly in smaller breeds. I would feed three times a day until your dog is about 4-5months old and then you can drop to twice a day...i wouldnt suggest just once a day...they are small dogs and need the nutrition.
Also, it might not hurt to carry Nutra-Cal. If you see that your puppy isnt eating as well as she/he should you can give it Nutra-Cal, basically a syrup-type supplement that has a lot of sugar and calories...not a substitue for regular puppy food, but it will give your pup a boost if needed. All you would need to give is about a pea-sized dropplett...I have yet to see a dog that doesnt like it.
Hope this helps, please feel free to ask any more questions
It really does depend on the size of the dog. I got away with just three times a day for Tequila and Kylie because they were bigger, but Ginger I fed more...more like 4-5 times a day. When I first got her she was under a pound and now at 9 months she is about 3lbs (will find out for sure tomorrow). I always left a bowl of food down just for her if she got hungry. I usually dont suggest free-feeding but I would get nervous if she didnt eat a lot.