Adult dogs should eat twice a day - morning and night. Puppies under 6 months of age (especially small breed ones) must be fed more often.
Most veterinarians (including myself) recommend feeding adult dogs twice a day. Some people feed their dogs once a day, but dogs that are only fed once a day can get hungry and also sometimes can be prone to bilious vomiting (vomiting caused by an empty stomach). Puppies need to eat more frequently and should be fed four times per day at first, and can slowly be weaned down to twice daily feedings by about six months of age.
Most dog food includes instructions for total daily caloric requirements - therefore you will take that amount and divide it by the number of feedings.
Very small breed puppies (think Chihuahuas, Yorkies, Pomeranians, and any “teacup” varieties”) will require much more frequent feedings in the first few weeks as they are prone to hypoglycemic episodes (low blood sugar). Due to their small size, they do not have the energy reserves of larger and older pups and therefore can quickly become hypoglycemic. This can be a medical emergency. Therefore, it is very important to feed your tiny pup every 2-3 hours during the day in the beginning. If your puppy becomes profoundly lethargic at any time, they should be immediately evaluated by a veterinarian as this could be a life-threatening emergency.