Quick answer: Most adult dogs sleep 12-14 hours per day — it's completely normal. Puppies and senior dogs sleep even more. Concern is warranted when sleep increases suddenly with other behavioral changes.
Puppies sleep 18-20 hours daily — their brains and bodies are growing rapidly. Adult dogs average 12-14 hours depending on breed and activity level. Giant breeds and senior dogs typically sleep more. Dogs also sleep more in cold, quiet, or dark environments.
Working breeds (Border Collies, Huskies, Malinois) tend to sleep less and need more stimulation to stay settled. Giant breeds (Great Danes, Mastiffs, Bernese Mountain Dogs) and brachycephalic breeds (Bulldogs, Pugs) sleep more and tire more easily. Breed baseline context is useful.
A sudden increase in sleep accompanied by reduced interest in walks, food, or play; difficulty getting up; or other behavioral changes may indicate illness, pain, hypothyroidism, or other conditions. If your usually active dog has become noticeably more lethargic over days or weeks, see a vet.
Sleeping a lot is normal for dogs. A sudden, noticeable change in a dog that was previously active is the signal to pay attention to — not the absolute number of sleep hours.