Why do human mostly give birth to single baby at a time?

Firstly, a woman usually only ovulates one egg each cycle, so twinning is always a rarity (either two eggs drop, or one splits into two). During reproduction, only one sperm is allowed to fuse with one female egg and then the entries of other sperms are prevented, because it can cause problem in the fertilization process. This can lead to improper growth or baby.

Other than that human babies are very vulnerable and depends on the mother for years, so to promote the survival rates, one baby is born at a time

There is also another factor at play. Human infants require quite a bit of care from their parents. Animals that have large litters are often far more self-sufficient when born. Human infants can't even crawl, let alone walk or run, for months or even years. As such, evolution dictated that survival is maximized for our species when there is only one baby at a time, as the overworked parents can give more required nurturing to one child at a time than five or six, and as such, that child is more likely to survive.