how do I know if my sim is having 1 or more babies?

I don't believe there is a way to fully tell what your sim is going to have. The sex and how many babies that are going to be born are automatically decided as soon as the sims woohoo and become pregnant. Adding on to the answer, in the Sims 3 there was a way to encourage if your sims was going to have a boy or a girl. If you wanted a girl you would have to eat watermelon etc, but like I said, the sex is decided as soon as your sims woohoo.