Here's why it's difficult to know:
* Individual dogs have different litter sizes: Some breeds naturally have larger litters than others.
* Puppies can be hidden: A mother dog might hide a small or weak puppy, especially if it's her first litter.
* Labor can be irregular: Labor can be long and drawn out, with puppies being born several hours apart. It's not uncommon for a puppy to be born after the mother seems to be done giving birth.
If you're concerned that the litter might not be complete, it's always best to contact a veterinarian. They will be able to provide the most accurate information and guidance.