In the human species, one male partner is enough for the sperm to fertilize the egg. The average cargo is just one child, half of its DNA from the mother and the other half from the father. But there are cases when the fertilized egg splits into more than one, and that's where twins and triplets and so on and so forth are, in a nutshell, possible.
But let's say that twins or triplets are the result of one mother having her egg fertilized by two or three genetically distinct sperm from two or three different male partners within a three-day limit. Any higher a number is not immediately fatal, but it can still increase the risk of complications during pregnancy or labor.
There are some species of our fellow mammals that have that kind of sexual mechanics. Indeed, some species of primates have one mother having children from more than one male partner. So why don't we have that flexibility? Would it make sense to even try to have that?
No mention of fertility drugs, please. They are not considered natural human physiology.