A thousand times over
יקוק אלקי אבותכם יסף עליכם ככם אלף פעמים ויברך אתכם כאשר דבר לכם
Hashem, the G-d of your forefathers, should increase you a thousand times over, and He should bless you as He said He would
Moshe, as part of his goodbye speech to the people, blessed them that they should increase a thousand times over. Then, he said that Hashem should bless them as He already said He would. Why did Moshe finish his personal blessing by referring to Hashem’s blessing? Rashi explains that the Jewish people, upon hearing Moshe’s blessing of the thousandfold increase, began to protest. “Moshe our teacher! How can you limit your blessing to just a thousand! Hashem’s blessing knows no limits, and He already promised us that we will be so numerous that we’ll impossible to count!” Moshe’s response was that this blessing was from himself, but for sure Hashem’s blessing is still in effect. However, this doesn’t really answer the question. If the thousandfold increase was Moshe’s personal blessing, why indeed did he limit it to a thousand?
When the Jewish people sinned with the Golden Calf, Hashem told Moshe that He’ll make him into a great nation, greater and mightier than the Jewish people. Hashem said the same thing when the Jews sinned in the incident of the spies. Now, the Torah doesn’t specify how much greater Moshe’s nation would have been than the Jews. However, our Sages tell us that Hashem’s attribute of Good is greater than His attribute of Bad by five hundred times. It would make sense then to say that Hashem’s intent was to make Moshe’s nation five hundred times greater in size than the Jewish people.
Now, even though this decree never came to fruition, as Hashem ultimately forgave the Jewish people, good tidings that come from Hashem are never annulled. Where then do we see this blessing fulfilled? Since Hashem blessed Moshe with this blessing twice, the two times five hundred comes out to a thousand. It turns out then that Moshe is blessed a thousand times greater than that of the Jewish people.
Moshe’s response now makes perfect sense. When Moshe wanted to bless the people to be fruitful and multiply, he told them that he was blessing them of his own. How much is his own? That they should grow a thousand times over. However, that’s not all that they’ll receive. Hashem already blessed them that they’ll be innumerable as the sand of the sea. That blessing is limitless.
 Based on the end of the Hafla’ah’s introduction to his commentary to Kesubos (commonly called Sefer Hafla’ah, although the author himself called it Sefer Kesubah)
 Deuteronomy 1:11
 Rashi ad. loc.
 Genesis 13:16
 Exodus 32:10
 Numbers 14:12
 Tosefta Sotah 4:1
 שאמרו חז”ל שכל דבור שיצא מפי הקב”ה לטובה אפילו על תנאי אינו חוזר. I’m not sure which Chazal the Hafla’ah is quoting. See Midrash HaGadol to Genesis 32:8, which is the closest I found