The first Mitzvah of the Torah and the first blessing given by God to Adam and Eve, is the commandment to (marry and) have children. Bereshit 1: 28 says: "And God blessed them [Adam and Eve] and God said to them, reproduce and multiply".
The Rabbis of the Mishna, discussed the Mitvza of having children is considered fulfilled.
Bet-Shamai said that the Mitzvah is fulfilled by having at least two boys: same as Adam and Eve, who at the beginning brought two boys to the world: Cain and Abel.
Bet-Hillel, however, says that the Mitzvah is not considered fulfilled until one has at least a boy and a girl. Because this is how God created human civilization: with one man and one woman. The matter was settled like Bet-Hillel.
There is an additional reason for this opinion: if you read carefully the words of the Torah, a husband and a wife must first "reproduce" themselves, in other words, have a replica of themselves (a man and a woman) leaving in this world the same number and kind of lives that one day will be taken away.
One more thing: although the Mitzvah is fulfilled by having one boy and one girl, the rabbis explained that it is a great merit to have as many children as one can. They deduced this guidance from the second word of God's commandment: "reproduce and multiply"."Reproduce" means bringing to the world a replica of yourselves. "Multiply" means "bring into this world more souls than yourselves".
Of course, this Mitzvah does not depend exclusively on our good will: its uniqueness consists that to fulfill this Mitzvah the spouses need to be granted God's blessing, as the Pasuk says: the blessing of having children.
