Seventy functions in the Hebrew Bible as the number of a complete assembly, and it does so consistently enough that the usage is clearly conventional.
The table of nations in Genesis 10 lists the descendants of Shem, Ham and Japheth, and the traditional count of the peoples named is seventy. That figure became the standard Jewish reckoning of the nations of the world, and it underlies the rabbinic idea of the seventy languages and the seventy guardian angels of the nations. It also stands behind the tradition that the seventy bulls offered across the days of Sukkot are offered on behalf of the seventy nations.
Seventy of Jacob's household go down to Egypt at Genesis 46:27 and Exodus 1:5. The Septuagint gives seventy-five, a discrepancy that Acts 7:14 follows, so the two numbers descend into different traditions.
Seventy elders ascend the mountain with Moses at Exodus 24:1 and 24:9, and at Numbers 11:16-25 seventy elders receive a share of the spirit that was on Moses, the foundational text for the later institution of the Sanhedrin, which numbered seventy-one with the head of the court included.
Seventy years of exile are set at Jeremiah 25:11 and 29:10, a figure Daniel 9 reinterprets as seventy weeks of years. Whether the seventy years is an exact count depends on which events are taken as its endpoints, and the ancient sources already treat it as approximate.
Psalm 90:10 gives the days of our years as threescore years and ten. The Hebrew Bible therefore uses seventy for a full lifetime, a full assembly, a full generation of exile and the full set of nations.