Generally, most adult dogs should eat at least twice a day. However, it's very important that the frequency can vary depending on the dog's age, breed, physical activity level, and health. Puppies, for example, may need three or four small meals a day, while older dogs may need more, smaller meals to aid digestion.
Here's a more detailed guide:
Puppies:
Puppies typically eat three or four times a day until they are 6 months old, according to a Purina guide.
Adult dogs:
Most adult dogs should eat twice a day, once in the morning and once in the afternoon or evening.
Senior dogs:
Older dogs may need more small meals per day to aid digestion, according to Happy Dog 
Breed and size:
Small breeds, especially puppies, may need to eat more frequently, albeit in smaller portions, according to Purina.