The most common Greek Septuagint text (referred to as the Septuagint B) says that there were 70 of the IsraElite’s ancestors that originally traveled down to Egypt during the terrible famine in the time of JoSeph.
This agrees with the Hebrew texts, which consistently say 70. However, Acts 7:14 recounts the tale and says that there were, not 70, but 75 of them.
So which is it, 70 or 75?
Well, a more ancient Greek text (known as Septuagint A, perhaps used by Luke when writing the Acts) says 75. This also agrees with Septuagint B’s version of Genesis 46:27 and at Exodus 1:5, which say there were 75.
So which number is correct?
Probably 75. Even if that’s wrong, such a minor deviation in numbers has little effect on our basic understanding of the Bible.