The passage in question is Genesis 46:8-27 which breaks from the narrative to offer yet another genealogy: “And these are the names of the children of Israel who came to Egypt…”
We have seen elsewhere that such interest in genealogies, dates, and ages were evidence of the Priestly writer’s hand. Yet this passage also evidences editorial reworking, possibly even done by a scribal hand during the recopying of the manuscript. In other words, within this single source there is a discrepancy pertaining to the number of male descendants from Jacob’s loins that went down to Egypt.
Verse 26 states that there were 66, while verse 27 states 70. One of these is an editorial correction most likely inserted during textual transmission. That seems to be the best hypothesis that fits the textual data.
If one counts up all the male descendants of Jacob listed in the passage, one arrives at the total of 70—thus verse 27: “All the persons of Jacob’s house who came to Egypt were 70.” However this is inaccurate; 4 of the males listed did not in fact go down to Egypt. For the original 70 mistakenly counts Judah’s sons Er and Onan. But they were killed by Yahweh earlier in the narrative—at least according to the Yahwist tradition (Gen 38). It is possible that the Priestly hand from which this genealogy was written was unaware of, or consciously wanted to suppress, this Yahwist story. If this were the case, this serves as yet another textual example of a later tradition reinterpreting or subverting an earlier tradition, both of which now exist in the Bible, sometimes verse-by-verse as in this example.
Two others, Joseph’s sons Manasseh and Ephraim, which are also listed among those who go down to Egypt, were also subtracted from the original 70 by the hand of this later editor given the fact that both these sons were born in Egypt and therefore cannot be counted among those who traveled down to Egypt. Subtracting these 4, we arrive at 66 males of Jacob’s household who went down to Egypt, verse 26.
It is difficult to say which number was the earlier, and which the latter. Some scholars have speculated that the number 70 was the earlier reading, especially if we recognize the legendary character of the number 70 in both Israelite and Canaanite literature. If the assignment was purely based on legend, then 70 best fits this aim, and additionally explains why we find the number 70 given in Ex 1:5 and Deut 10:22. It is quite possible, therefore, that the number 66 came as the result of a scribe who aimed for more precision, or who saw the error among those who were being counted in the 70.
To complicate matters further, Acts 7:14 stipulates that there were 75 in all. Where does this number come from? There are in fact a couple of manuscripts that attest the number 75, one of which is from Qumran (4QExod), and the other is from the LXX where Genesis 46:27 reads 75. Both of these manuscripts suggest that in the pre-Christian era there were two Hebrew manuscripts that had 75, one of which served as the text for the translation of the Greek LXX. It is this manuscript, the Greek Torah, that the author of Acts would have most likely had access to.