We know that there are
33 by Leah
16 by Zilpah
14 by Rachel
7 by Bilhah
= 70

HOWEVER, a few points:

1. Genesis 46:15 says “These sons AND daughters…” so we must assume that Dinah is counted [she's mentioned just before in verse 15]. This brings us to 34. To get back down to 33, we subtract Ohad son of Simeon. Why? Because his name does not appear in the Septuagint [very early Greek translation]. Nor does his name appear in the similar lists at Numbers 26:12-13 or 1Chr. 4:24. So we bring Leah back down to 33 children. That gives us our 70 again.

Now, verse 26 tells us that only 66 went to Egypt, so we need to drop four. This is actually not as difficult as it seems. Er and Onan are dead [see chapter 38.] Perez’s two sons [Hazron and Hamul] could not POSSIBLY have been born yet at this time, since Perez son of Judah had to be VERY young at this time. So why are they counted in 46:12? Notice the wording:

“The sons of Judah: Er, Onan, Shelah, Perez and Zerah [but Er and Onan had died in the land of Canaan]. The sons of Perez: Hezron and Hamul.

The solution, I believe, is to simply extend the parenthesis about Er and Onan to include the note about Hezron and Hamul. In fact, THE HEBREW WORDING HERE IS DIFFERENT FROM THAT USED IN THE REST OF THE CHAPTER.

So by dropping two dead sons [Er and Onan] and leaving to the side Hezron and Hamul [who couldn't have been born] we arrive at the 66 children mentioned in verse 27. This solves two issues:

1. How could Hezron and Hamul have gone to Egypt when their father must have been a young child, and

2. Why don’t the numbers add up to 66.

NOW, to get 75, we simply take the 66 and add the nine sons of Joseph [see the Septuagint of verse 27].

66 + 9 = 75.

Not only does this match Stephen, it also explains the issue in verse 26 in the Hebrew, which says 66 + 2 = 70. 66 + 9 = 75 makes much, much more sense, of course.

The underlying defense of inerrancy is that Stephen was wrong and Acts accurately reports his mistake.

By mentioning Joseph’s family and Jacob, I was merely suggesting a group of five that he might have thought needed including in the total. Here’s a list of possible ‘fivesomes’ that might account for Stephen’s mistake:

1) Joseph’s family + Jacob
2) Joseph’s family + Dinah
3) Joseph’s family + Shaul (possibly the bastard of Simeon).
4) Joseph and his sons + Er and Onan (these two died on the way).
5) Joseph and his sons + Jacob and Shaul
6) Joseph and his sons + Jacob and Dinah
7) Joseph and his sons + Shaul and Dinah

In the end, it doesn’t really matter which of these fivesomes Stephen had in mind (perhaps he had in mind another fivesome altogether, or perhaps he was just misremembering the number or repeating an incorrect number he learned by rote). The doctrine of inerrancy asserts the inerrancy of Scripture, not of Stephen.