Seventy is made of two perfect numbers, seven- representing perfection and ten- representing the ten commandments and the completeness of The Holy One’s law, symbolizing perfect spiritual order carried out with all power.
Midrash: The Holy One, Who has seventy Names, gave the Torah, which has seventy names, to Israel, which has seventy names (Numbers 11:16), and which originated from seventy people who went down to Egypt with Jacob (Genesis 46:8-27), and was chosen from among seventy nations (Genesis chapter 10), to celebrate seventy holy days in the year (52 Sabbaths and 18 festivals, including the Intermediate Days between Passover and Succot). The Torah was transmitted to seventy elders (Midrash Yelamdeinu), and safeguarded by the Sanhedrin of seventy Sages (Numbers 11:16) … There are seventy facets to the Torah (Zohar, Genesis 36), which was translated into seventy languages to make it understandable to the seventy nations (Sotah 32a), and was engraved on seventy stones after Israel crossed the Jordan (Deuteronomy 27:8) on their way to the Holy Land. Jerusalem, which has seventy names, built the Temple with seventy pillars. On Succot, seventy sacrifices were offered (Numbers 29:13-34) for the sake of the seventy nations of the world who have seventy representatives among the heavenly angels.
Judah Loew ben Bezalel, The Maharal of Prague: The number seven represents the entirety of this natural world, which was created in seven days (six days of creation, completed on the Sabbath). Any number times ten represents its expanded full potential – seventy of something represents all the potential facets of that thing in the natural world.
Rabbi Dovid Feinstein, the son of the late Moshe Feinstein: The seventy languages used by Moses parallel the seventy facets of Torah; each speaks to one of the seventy characteristics with which G-d has populated the world. The number seventy symbolizes that every national trait can be harnessed for holy purposes.
Mark Twain on his seventieth birthday: It is the time of life when you arrive at a new and awful dignity; when you may throw aside the decent reserves which have oppressed you for a generation, and stand unafraid and unabashed upon your seven-terraced summit and look down and teach – unrebuked. You can tell the world how you got there. It is what they all do. I have been anxious to explain my own system this long time, and now at last I have the right.