Stephen got the number 75 wrong.

Gen 46:26 ESV All the persons belonging to Jacob who came into Egypt, who were his own descendants, not including Jacob's sons' wives, were sixty-six persons in all.

Act 7:14-15 ESV And Joseph sent and summoned Jacob his father and all his kindred, seventy-five persons in all. (15) And Jacob went down into Egypt, and he died, he and our fathers,

Genesis has 66 people coming into Egypt but Stephen in Acts has 75 people, why? The answer is that Stephen was quoting from his Greek OT the Septuagint (LXX)¹.

Here is what Barnes says on this (Albert Barnes - Notes on the bible):

Threescore and fifteen souls - Seventy-five persons. There has been much perplexity felt in the explanation of this passage. In Gen_46:26, Exo_1:5, and Deu_10:22, it is expressly said that the number which went down to Egypt consisted of 70 persons. The question is, in what way these accounts can be reconciled? It is evident that Stephen has followed the account which is given by the Septuagint. In Gen_46:27, that version reads, “But the sons of Joseph who were with him in Egypt were nine souls; all the souls of the house of Jacob which came with Jacob into Egypt were seventy-five souls.” This number is made out by adding these nine souls to the 66 mentioned in Gen_46:26. The difference between the Septuagint and Moses is, that the former mentions five descendants of Joseph who are not recorded by the latter. The “names” of the sons of Ephraim and Manasseh are recorded in 1Ch_7:14-21. Their names were Ashriel, Machir, Zelophehad, Peresh, sons of Manasseh; and Shuthelah, son of Ephraim. Why the Septuagint inserted these, it may not be easy to see. But such was evidently the fact; and the fact accords accurately with the historic record, though Moses did not insert their names. The solution of difficulties in regard to chronology is always difficult; and what might be entirely apparent to a Jew in the time of Stephen, may be wholly inexplicable to us.

1 Septuagint - What is It? Septuagint (sometimes abbreviated LXX) is the name given to the Greek translation of the Jewish Scriptures. The Septuagint has its origin in Alexandria, Egypt and was translated between 300-200 BC. Widely used among Hellenistic Jews, this Greek translation was produced because many Jews spread throughout the empire were beginning to lose their Hebrew language. The process of translating the Hebrew to Greek also gave many non-Jews a glimpse into Judaism. According to an ancient document called the Letter of Aristeas, it is believed that 70 to 72 Jewish scholars were commissioned during the reign of Ptolemy Philadelphus to carry out the task of translation. The term “Septuagint” means seventy in Latin, and the text is so named to the credit of these 70 scholars.

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How Many Went to Egypt with Jacob? Eric Vestrup