The Mitzvah of Having Children
Genesis 1:28, or according to some opinions, 9:7, is the source for the Torah commandment to "be fruitful and multiply". Technically, this commandment applies to men and not women. However, since today a man may have only one wife, she is an essential partner in enabling her husband to fulfill the commandment.
The Torah commandment is defined by halacha as having one son and one daughter who themselves reproduce. There also exists a rabbinic admonishment to continue to procreate even after fulfilling the quota mandated by the Torah. This is expounded from the verse in Ecclesiastes 11:6 "In the morning sow your seed, and in the evening do not desist." The exact definition and parameters of the rabbinic obligation are the subject of much discussion, and a couple should consult with their rabbi.