Some organisms like dog and rates give birth to more than one offspring whereas humans give one offspring
Answers
According to the theory of the selfish gene all genes “want" to propagate themselves at the expense of all other genes, so all fathers, human or otherwise would prefer the female they have just fertilised to have as many babies as possible. This if fine if you are a fish, a bee or a termite. If you are a female, your genes also want to be propogated as much as possible. If you are a female and a mammal esthere are other considerations for your womb can only accommodate a limited number of foetuses before the size and weight will kill both the mogher and the offspring
This problem can be solved by giving birth to the young when they are small and undeveloped. So America opposums and the common housemouse can easily have more than a dozen offspring and so in fact can the dog or the pig. The young are born in nests and dens where fhey can grow in safety nourished by milk. The limiting factor on litter size becomes the number of teats that the mother has and even this can be overcome if other mothers are conscripted as wet nurses. Large herbivores however tend to give birth to single babies that are up and running a few hours after birth. Elephants, giraffes, elands, camels, zebras and wildebeest typically have singletons. Twins are not impossible with large herbivores and domestic cattle occasionally have twins and sheep frequently do with a fair number of triplets as well. Twins are always smaller than singletons which has disadvantages and advantages. The obvious disadvantage is that a twin being smaller will be weaker and slower and most likely to fall victim to a predator and so the tendency to have twins will be gradually deleted from the genome. With domestic sheep there is an advantage both to the farmer and to the mother. Predation has been removed from the farm and the pasture is if anything too nutritious. A ewe will gorge herself and her lamb or lambs will grow fat in the womb. Farmers know too well that singletons can be too big and can get stuck in the birth canal during birth causing the death of the lamb, the ewe or both. Twin lambs are smaller and invariably give many fewer problems at birth. New Zealand farmers selectively breed ewes that produce twins. With cattle twins are not desirable because a certain percentage of them will be male and female paired twins. In this instance the male calf always masculinizes his sister in the womb and she is born as a sterile female called a freemartin.
There are other factors determining the size of litters. In a healthy ecosystem vegetation, herbivores and predators must all stay in balance. If for example the season is good and there is a heavy set of acorns and beech mast,there will be abundant food for mice, rats and voles who will increase their population on the better diet. If the carnivores do not match the expansion of herbivore numbers thete will be a plague resulting in overgrazing and eventually starvation for both herbivores and predators. For predagors to have large litters is an advantage as it allows them to keep exploding prey populations in check, but it an insurance policy with a hefty premium. In good years most cubs will survive in bad years most will die.
In the human situation we have helpless babies with very large heads. Twins might seem like a good idea. Once born however the cbildren need feeding for the next fourteen years and when food is scarce having two mouths to feed rather than one could prove disastrous. After the black death or similar plagues a higher replacement birth rate would be a good thing, but whenever famine was more prevalent than the other horses of the apocalypse, smaller familes would be better. It is conflicting factrs such as these which have kept multiple births possible with humanx but relatively rare.