Genesis 46:27
27 And the sons of Joseph, which were born him in Egypt, were two souls: all the souls of the house of Jacob, which came into Egypt, were threescore and ten.
Acts 7:14
14 Then sent Joseph, and called his father Jacob to him, and all his kindred, threescore and fifteen souls.
Stephen says in Acts that there were 75 that came to Egypt with Jacob, whereas in the account in Genesis it says there were 70.
The 70 descendants in Genesis 46 can be counted, because they are all listed.
They are:
Genesis 46:15 counts the total of these (Leah's descendants) to be 33. That means that Onan and Er are counted, and Dinah is not.
However, since Onan and Er are dead, they will not be coming into Egypt with Jacob. Dinah, however, will be. That brings the count to 33 - 2 + 1 = 32.
Continuing:
Genesis 46:18 says that the total of these (Zilpah's descendants) was 16, as expected. So, the count is now 32 + 16 = 48.
Next:
Genesis 46:22 says that the total of these (Rachel's descendants) was 14. So, the total is now 32 + 16 + 14 = 62.
However, note that Joseph and his sons are already in Egypt, therefore they do not have to come into Egypt with Jacob and the rest of the family. So Joseph and his two sons are subtracted from the total: 62 - 3 = 59.
Finally:
Genesis 46:25 says that these were Bilhah's descendants, and they total 7. So, the total is now 32 + 16 + 11 + 7 = 66.
Genesis 46:26 then says:
Genesis 46:26
26 All the souls that came with Jacob into Egypt, which came out of his loins, besides Jacob's sons' wives, all the souls were threescore and six;
The above states that 66 people are counted as officially coming into Egypt with Jacob, as expected.
The next verse says:
Genesis 46:27
27 And the sons of Joseph, which were born him in Egypt, were two souls: all the souls of the house of Jacob, which came into Egypt, were threescore and ten.
This is the total, which includes Joseph and his two sons, and Jacob himself - 66 + 3 + 1 = 70.
So, there is no internal issue within Genesis 46 itself. The count of 70 is rather straightforward.
How then did Stephen arrive at 75?
Recall that the verse references all of Jacob's kindred:
Acts 7:14
14 Then sent Joseph, and called his father Jacob to him, and all his kindred, threescore and fifteen souls.
This number would need to include Er and Onan, who were not included in the original 70 from Genesis 46. So that would bring the count to 70 + 2 = 72.
One way to add 3 more would be to add Leah, Zilpah, and Bilhah - 3 out of Jacob's 4 wives - excluding Rachel only because she died giving birth to Benjamin. This would be assuming both that his other wives were still alive, and that they accompanied him to Egypt.
This may seem strange, as two dead people, Onan and Er, were evidently included in his count. However, Onan and Er were obviously related by blood to Jacob, whereas Rachel was not. Therefore, perhaps Stephen saw it fit to count them, and added the three other wives by virtue of the fact that their deaths had not been noted by the time Jacob actually came into Egypt.
Another way to explain Stephen's count of 75 would be that the Septuagint in Genesis 46:27 reads 75. It includes two descendants from Ephraim, and three from Manasseh (Genesis 46:20), some of whom are mentioned in 1 Chronicles 7:14-21. The Septuagint would have been the popular translation of the Old Testament at the time, and so Stephen's reference may have been obvious to the contemporaries that Stephen was addressing.
There are ways to reconcile the two tallies of Jacob's descendants, however, Stephen's tally of 75 given in Acts chapter 7 is open to more interpretation than the tally of 70 given in Genesis 46.