There is a dispute between Beit Shammai and Beit Hillel as to what constitutes a fulfillment of the commandment to "be fruitful and multiply" (peru u'revu).
מתני' לא יבטל אדם מפריה ורביה אלא א"כ יש לו בנים ב"ש אומרים שני זכרים וב"ה אומרים זכר ונקבה שנאמר זכר ונקבה בראם
MISHNAH. A MAN SHALL NOT ABSTAIN FROM THE PERFORMANCE OF THE DUTY OF THE PROPAGATION OF THE RACE UNLESS HE ALREADY HAS CHILDREN. [AS TO THE NUMBER]. BETH SHAMMAI RULED: TWO MALES, AND BETH HILLEL RULED: MALE AND A FEMALE, FOR IT IS STATED IN SCRIPTURE, MALE AND FEMALE CREATED HE THEM. (Soncino translation; capitals in original)
The law is codified by Rambam in accordance with Beit Hillel:
כמה בנים יהיו לאיש ותתקיים מצוה זו בידו זכר ונקבה שנאמר זכר ונקבה בראם היה הבן סריס או שהיתה הבת אילונית לא קיים מצוה זו
How many children is it necessary for a man to have fathered to be considered to have fulfilled this mitzvah? One boy and one girl, as [implied by Genesis 5:2]: "He created them, a male and a female." If the son was a saris or the daughter an aylonit, he is not considered to have fulfilled this mitzvah. (Chabad.org)
R. Avraham Ben HaRambam has a responsum in which he clarifies his father's view about the mitzvah of marriage. In this responsum he distinguishes between the mitzvah of marriage and the mitzvah of peru u'revu. Defining the latter, he writes:
ומצות פריה ורביה מצוה אחרת היא שמצות פריה ורביה כשיהיה לו בן או בת קיים המצוה
And the commandment of being fruitful and multiplying is a separate commandment; [regarding] the commandment of being fruitful and multiplying, once he has a son or a daughter he has fulfilled the commandment.
This is somewhat surprising, as R. Avraham appears to state that the commandment is fulfilled with just one child, boy or girl. This follows neither tannaic view, and is against the ruling of his father.
Torah Chazal and Science p. 87
In Rabbeinu Avraham's case, by contrast, we almost never find him disagreeing with his father on halachic issues in his published writings.
How are we to explain this statement of R. Avraham? Does he actually hold that only one child is necessary, against both views in the Mishnah and against the codification of his father? Was he simply being imprecise in his description of the mitzvah? Do any subsequent authorities deal with this puzzling passage?
My own intuition was that there might have been a typo/scribal error, and the words בן או בת really were בן ובת (i.e. an extra א was added) with the original meaning "a son and a daughter". Indeed I found one contemporary sefer that just casually said:
נ' דצ"ל בן ובת
It appears that it should say "a son and a daughter".
But I don't have any manuscript evidence for this.