The first Mitzva of the Tora and the first blessing given by haShem to humankind is to marry and have children. Genesis 1:28 says: "And God blessed them [Adam and Eve] and God said to them, reproduce and multiply, fill the earth and be its master".
The Rabbis of the Mishna debated on the point at which the Mitvza of having children is considered fulfilled.
Bet-Shamai said that this Mitzva is fulfilled by having 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 this Mitzvah is not considered fulfilled until one has at least a boy and a girl. Because this is how God created the first prototype of humankind: one man and one woman.
The Rabbis resolved the discussion ruling like Bet-Hillel.
Why? Among other things because the first word of this commandment peru literally means: to "reproduce". A husband and a wife must first produce a replica of themselves, i.e., a man and a woman. Leaving in the world the same number and kind of lives that God originally planted on this planet.
One more thing: although the Mitzvah of peru urbu is fulfilled by having a boy and a girl the Rabbis said that it is a great merit to have as many children as one can. This guidance can be learned from the second word of this Mitzva: urbu which means: "multiply". The first word "reproduce" means bringing to the world a replica of yourselves, the second word "multiply" means to increase yourselves by "bringing into this world more lives than yourselves".
Obviously this Mitzva is not exclusively in our hands. To fulfill this Mitzva one first has to get married. And then the spouses need the most beautiful gift of HaShem: the blessing of having children.
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