In the female pig reproductive cycle, more number of eggs are released during one ovulation cycle. That is why, the number of offsprings born is more. The father of the litter is same single animal only. Because the process of fertilization of sperms with the eggs released happen in a time duration of few hours only.
In case of humans as well, the father is same. This is because in human reproductive cycle, only one egg is released majorly, accept the case of fraternal twins where two eggs are released and both are fertilized by two different sperms, but again from the same father.