The age of the dog when breeding plays a key role in the size of the litter. The older the prey, the smaller the litter, especially in larger breeds. Mothers raised 2 to 5 years old tend to have the largest litters. Despite the age of the prey, the first 2 litters will generally be smaller than the later ones.
As mentioned above, litter size varies based on several factors, but for the sake of discussion, we'll assume you have around five puppies in each litter. In 2004, a Neapolitan mastiff named Tia became the mother of the largest litter ever documented, when she gave birth to 24 puppies by cesarean section. Large dogs tend to have larger litters, while small dogs tend to have smaller litters, most obviously because the size allows larger dogs to have more puppies safely. And, every time a dog becomes pregnant, she is more likely to have even more puppies with the next calf.
You can't put two parents who previously produced large litters and expect puppies to produce large litters two years later as well. The average number of puppies in a litter of German Shorthaired Pointer ranges from 8 to 12 puppies, but smaller and larger litters are expected. Breeding in limited genetic groups, such as dogs that come from smaller breeding groups where genetic diversity is very limited, is known to produce a (much) smaller litter size of puppies. A decrease in average litter size of 0.4 puppies would be expected for litters conceived with AI with fresh semen and 1.3 for AI with frozen semen, both compared to natural mating.
Like the dog's age, the first analysis of the data showed no significant change in litter size according to the mating method. Also having mating at the end of the fruiting period of the female dog and not from the beginning is a factor that leads to having more puppies. Just because my dog, who is a Jack Russell Chihuahua and Boston Terrier, gave birth on his own to a litter of 12 puppies but only 11 survived. We cannot find a reliable study that has sought to determine the most fertile breed, but it is surely one of the largest, such as one of the mastiffs, the Irish wolves or the Great Danes.
But understand that this is a statistical correlation, not a way to predict the number of puppies your individual dog will have. Towards the end of your dog's pregnancy, the vet will likely be able to feel the mother's belly or take an X-ray to determine an “exact number of puppies in their belly” (although it can be easy to miss one of the puppies, so you'll never know for sure until the little wiggles start to come out). Miniature breeds generally produce litters of 3.5 puppies, while large breeds generally produce 7.1 puppies per litter.