Tallying the numbers in the parsha
First let's go over the rest of the numbers, though. 46:18 counts up the children of Zilpah as 16. 46:22 counts up the children of Rachel as 14. These include Binaymin, his 10 sons, and Yosef, and his 2 sons. 46:25 tallies the children of Bilha as 7. (Interesting note: Hakthav vehakabal cites a GRa that points out that Rachel and Leah each had twice as many people assigned to them as their corresponding shifchos.) Anyway, 33 plus 16, plus 14 plus 7 adds up to 70. However, 46:26 then counts up differently. The figure it gives is 66 as the total number of peopel who came down to Egypt. This is followed by the reminder that Yosef and his 2 sons wer already in Egypt.
To get to 66, we have to subtract 3 from the 14 names ascribed to Rachel's sons plus one other? So who is that one? According to Rashi, it would be Yocheved who did not count as one who left E"Y but did count in the total of 70. However, I would suggest another possibility that is also based on Midrash. Yosef's wife, Asnath, was said to be the daughter of Dina and Schem. She was sent off with a type of protective amulet placed upon her by Yaakov. She was adopted by Potiphar and his wife, which is why she is called their daughter. But, in fact, she is among the descendants of Leah, for her mother, Dina, is explicitly counted in the total.
Counting Asnath as both the unnamed 33rd for the children of Leah and one of the 4 needed to add up to 70 after the count of 66 fits very well. She would not be named explicitly because she was illegitamete and was not brought up among the family of Yaakov. However, she rejoined the family as the wife of Yosef. Though she was one of Leah's descendants, she did not travel with them, but perforce left E"Y many years earlier. Then the division of 66 for those coming would leave 4 all of the household of Yosef -- himself, his two sons, and his wife, who was also his half niece-- to add up to the total of 70.