I just discovered that many Rishonim address this question. For example:
Tosafos to Taanis 11a s.v. אסור לאדם
אסור לאדם לשמש מטתו בשני רעבון - ואם תאמר הרי יוכבד נולדה בין החומות ואותו העת עת רעב היה וע"כ שימשו מטותיהן בשני רעבון ויש לומר דלכ"ע לא הוי אסור אלא למי שרוצה לנהוג עצמו בחסידות ויוסף לא שימש אבל שאר בני אדם שימשו
If you ask, Yocheved, ...
To quote part of R. Hershel Schachter's article on the topic:
It is now several years into their marriage, and our couple has already been blessed with a son and a daughter. What now? The Talmud tells us in the name of Rabbi Yehoshua that even after one has fulfilled the biblical obligation of Pru U'rvu, he is still required to continue to have children ...
While he does not make any connection to the view of R. Avraham, R. Yosef Qafih cites a view of R. Sa'adia Gaon that also considers the commandment to only require one child (at least nowadays).
Commentary to Hilchot Ishut Chapter 15 Note 10
וכתב מבשר הבבלי בשם רס"ג דמה שבא בתורה שבע"פ דקיום מצות פריה ורביה
בשני בנים או בן ובת דוקא באותן הדורות שהיו ...
R. Eliezer Melamed writes in Peninei Halakhah, Simhat HaBayit uVirkhato 5:8 (Hebrew) as follows:
According to R. Yosef Karo, Beit Din can force a twenty year old man to get married either by beating him (Rif, Rambam) or by rebuking or fining him (Tosafot, Rosh) until he agrees to marry.
In practice, this would only happen in rare cases, such as if a man was ...
Gur Aryeh answers that unlike animals humans have free choice and may choose not to procreate. Therefore they needed an extra Bracha.
Tzeidah LDerech cites another answer that the Bracha was needed because humans go to war with each other, resulting in mass casualties.
Rav Hirsch on Bereishis 1:28 says that the ברכה for human beings is to enable them to perform the mission that Hashem has given them. It is not just a physical action or set of actions, but the moral and spiritual growth of the free willed human being.
There, in the unfree living creatures, as already remarked thereon,
with the giving of the blessing, i....
Let's stick with Sefer HaChinuch 1, explaining the goal of this Mitzvah:
משרשי מצוה זו, כדי שיהיה העולם מישב , שהשם ברוך הוא חפץ בישובו, כדכתיב: (ישעיהו מה יח) לא תהו בראה לשבת יצרה. (גיטין מא, ב במשנה)
The the reason behind this commandment that the world should be settled because God wants the world to be settled, as it says (Isaiah 45:18), "I did ...
Not a concrete source, but there is well known information about Rav Papa’s children:
חַנִינָא בַּר פַּפָּא, רַמִי בַּר פַּפָּא, נַחְמָן בַּר פַּפָּא, אַחַאי בַּר פַּפָּא, אַבָּא בַּר פַּפָּא, רַפֽרַם בַּר פַּפָּא, רָכִיש בַּר פַּפָּא, סוֹרְחָב בַּר פַּפָּא, אַדָא בַּר פַּפָּא, דָרוֹ בַּר פַּפָּא
This was not even all of his children per se (See here). ...
I would suggest that the distinction here is between things that were commanded to "Jews", and things that were commanded to everyone. Let's first look at the actual wording of Rashi's comment:
אמר ר' יצחק לא היה צריך להתחיל התורה אלא מהחדש הזה לכם שהיא מצוה ראשונה שנצטוו ישראל
R. Isaac said: The Torah need not have started but from "this ...
Maybe Sarah did have more children with Abraham. The Torah says:
"-V'Avraham zaken ba bayyamim, v'Hashem berach et Avraham bakol -- And Abraham was old, and well advanced in days, and the Lord had blessed Abraham with everything." [Gen. 24:1]
The Talmud asks: What is meant by 'with everything' [bakol]? In the fact that he had a daughter and her name ...
Sarah having a single child at 90 was a supernatural miracle. Having multiple children afterwards would have been multiple supernatural miracles. Since G-D already 'accomplished His mission' of continuing Abraham's progeny through Sarah with that one miracle, there was no need for Him to continue doing miracles by giving Sarah more children.
Avraham kept having children since it's important for a person to keep having children even after having fulfilled the Mitzvah of Peru Urvu. As the Gemmorah (Yevomos 62b) learns that a person must procreate through his whole life:
דתניא רבי יהושע אומר נשא אדם אשה בילדותו ישא אשה בזקנותו היו לו בנים
בילדותו יהיו לו בנים בזקנותו ... Rabbi Yehoshua says: ...
About those 5 kids per family... there are other sources that counter this, and at least some of the males weren't married.
For new children that will be born:
"לפי שהדרך גורמת לשלשה דברים ממעטת פריה ורביה וממעטת את הממון
וממעטת את השם"
A journey lessens three things: reproducing, wealth and reputation.
(Rashi Bereshit 12:2)
This question was raised by R. Samuel Eidels in his commentary to this passage. He explains it based on another Talmudic passage (Bava Batra 116a) which explains that King David was described as "lying with his forefathers" while Yoav was described as "dead" because David left behind a worthy son while Yoav did not.
Thus, here Isaiah was telling Hezekiah ...