There are a couple of possible ways to reconcile this, both within the established Rules behind resolving alleged Biblical discrepancies used by Apologists.
- Copyist errors.
The first comes by clarifying what "inerrancy" means, to debunk a straw-man argument that such a discrepancy in our current Bible is an issue in the first place. For an in-depth dive into the doctrine of inerrancy, see From a Fundamentalist standpoint, what does the phrase “Inspired, infallible, inerrant Word of God” mean?.
- Only the original manuscripts are considered inerrant, and we fully expect there to be copyist errors and other discrepancies in our current version.
- We can overcome the "So how do we know we can trust the Bible if we know it contains copyist/translation errors" question by understanding that we have many, many, manuscripts to compare, and even though many discrepancies do exist between the manuscripts, none of them are doctrinally significant. More at What is “Manuscript Evidence” and how is it useful?
So, the first obvious possibility is a copyist or translation error in some of the accounts somewhere along the way. That's what's proposed at the "Contradictions in the Bible" website.
Again, going back to the non-straw-man version of inerrancy, inerrancy does not mean hyper-literalism. (See the link above for more.)
- It's entirely possible that the authors were counting different sets of people. Say you're a reporter writing about an invasion that involved 1000 infantry men and two generals. You may record this as 1000 soldiers storming the hill, and another may record 1002. (Yeah, that's a bad analogy, but it sets up the argument used in this Apologetics Press article on the subject.
Similar to how a person truthfully can give different degrees for the
boiling point of water (100° Celsius or 212° Fahrenheit), different
figures are given in the Bible for the number of Jacob’s family
members who traveled into Egypt. Stephen (in Acts 7:14) did not
contradict the Old Testament passages where the number seventy is
used; he merely computed the number differently. Precisely how Stephen
calculated this number is a matter of speculation. Consider the
- In Genesis 46:27, neither Jacob’s wife (cf. 35:19) nor his concubines is included in the seventy figure.
- Despite the mention of Jacob’s “daughters and his son’s daughters” (46:7), it seems that the only daughter included in the “seventy” was
Dinah (vs. 15), and the only granddaughter was Serah (vs. 17).
- The wives of Jacob’s sons are not included in the seventy (46:26).
- Finally, whereas only two descendants of Joseph are mentioned in Genesis 46 in the Masoretic text of the Old Testament, in the
Septuagint, Joseph’s descendants are calculated as being nine.
Taking into consideration how many individuals were omitted from “the
seventy persons” mentioned in the Old Testament, at least two possible
solutions to this alleged contradiction may be offered. First, it is
possible that Stephen included Jacob’s daughters-in-law in his
calculation of seventy-five. Jacob’s children, grandchildren, and
great-grandchildren amounted to sixty-six (Genesis 46:8-26). If Jacob,
Joseph, and Joseph’s two sons are added, then the total number is
seventy (46:27). If, however, to the sixty-six Stephen added the wives
of Jacob’s sons’, he could have legitimately reckoned Jacob’s
household as numbering seventy-five, instead of seventy. [NOTE: Jacob
is listed by Stephen individually.] Yet, someone might ask how
sixty-six plus “twelve” equals seventy-five. Simple—not all of the
wives were included. Joseph’s wife obviously would not have been
calculated into this figure, if Joseph himself were not. And, at least
two of the eleven remaining wives may have been deceased by the time
the family journeyed to Egypt. We know for sure that Judah’s wife had
already died by this time (Genesis 38:12), and it is reasonable to
conclude that another of the wives had passed away as well. (In all
likelihood, Simeon’s wife had already died—cf. Genesis 46:10.) Thus,
when Stephen stated that “Joseph sent and called his father Jacob and
all his relatives to him, seventy-five people” (Acts 7:14),
realistically he could have included the living wives of Joseph’s
brothers to get a different (though not a contradictory) number.
This last article also addresses the discrepancy between manuscripts, and discusses more issues with the account. It's worth a read, as is this one from Tekton Apologetics.