How to reconcile the number of people that went down to Egypt with Jacob?

Sure there is all you have to do is add Joseph's wife and children and Benjamin who Joseph had kept with him.

The seventy are the ones which came to Egypt Jacob's family then you add Joseph his wife and children plus Benjamin who was already in Egypt and that gives you the seventy five Joseph's family.

According to When Critics Ask by Norman Geisler and Thomas Howe, Stephen was probably quoting from the Septuagint (the Greek version of the OT), which stated 75, instead of the Hebrew version, 70.

The answer goes on to state, "Jacob has twelve sons. Adding Jacob's grandsons and great-grandsons, the total was 66. Adding Ephraim and Manasseh who were born to Joseph in Egypt, the total is 68. When you add Jacob and his wife the total is 70, as the Hebrew records. The Septuagint, however, starting with Jacob's 12 sons, added Jacob's grandsons and great-grandsons for a total of 66. Then, it added the seven additional descendants of Joseph who were probably sons of Ephraim and Manasseh who were born to Joseph's sons some time after the migration of Jacob to Egypt, but before Jacob died. The Septuagint also omitted Jacob and his wife. This makes a total of 75 as Stephen mentions in the Acts passage."

There are a couple of possible ways to reconcile this, both within the established Rules behind resolving alleged Biblical discrepancies used by Apologists.

1. 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?.

Short version:

• 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.

2. Counting differently.

   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 following:

• 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.

Note:

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.