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. A "score" is 20.
Let's lay this out and find out the source of - and solution to - the alleged discrepancy.
We can go back into Genesis 46 and actually count the descendants since they are all listed.
We have:
Genesis 46:15 counts the total of these (Leah's kids) 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 our count to 33 - 2 + 1 = 32.
Continuing, we have:
Genesis 46:18 says that the total of these (Zilpah's kids) was 16, as expected. So we are up to 32 + 16 = 48 at the moment.
Next, we have:
Genesis 46:22 says that the total of these (Rachel's kids) was 14, again, as expected. So we are up to 32 + 16 + 14 = 62.
However, note that Joseph and his sons are already in Egypt, therefore they don't have to come into Egypt with Jacob and the rest of the family. So we'll subtract 3 for the time being: 62 - 3 = 59.
Finally, we have:
Genesis 46:25 says that these were Bilhah's children, and they total 7. That means we are at 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;
That all makes sense. 66 people are counted as officially coming into Egypt with Jacob.
That brings us to the very next verse, which 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.
Now we arrive at 70, as we add Joseph and his 2 sons, which as mentioned before, were already in Egypt. 66 + 3 = 69.
Then, adding Jacob himself rounds us out at 70.
So can the numbers in Genesis 46 be a bit difficult to understand? Yes, but I think I've done a reasonable job simplifying it. There's no issue within Genesis 46 itself.
How then did Stephen arrive at 75?
Recall that the verse lists all 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 means we are at 70 + 2 = 72.
The only way I can think to add 3 more would be to add Leah, Zilpah, and Bilhah - 3 out of Jacob's 4 wives - excluding Rachel because she died giving birth to Benjamin.
This would be strange though, given the fact that he counted two dead people already in Onan and Er.
However, Onan and Er were obviously related by blood to Jacob, whereas Rachel was not. Therefore, Stephen saw it fit to count them, and added the three other wives by virtue of the fact that they were alive when Jacob actually came into Egypt.
Does this have to contradict? No, it doesn't. You can't definitively say one way or the other what exactly Stephen had in mind by saying 75. From his perspective, it was 75, and I think my solution as to how he got there is plausible.