Abraham's female partners

Question

The Torah says:

וְלִבְנֵ֤י הַפִּֽילַגְשִׁים֙ אֲשֶׁ֣ר לְאַבְרָהָ֔ם נָתַ֥ן אַבְרָהָ֖ם מַתָּנֹ֑ת וַֽיְשַׁלְּחֵ֞ם מֵעַ֨ל יִצְחָ֤ק בְּנוֹ֙ בְּעוֹדֶ֣נּוּ חַ֔י קֵ֖דְמָה אֶל־אֶ֥רֶץ קֶֽדֶם׃ -- but to Abraham’s sons by concubines Abraham gave gifts while he was still living, and he sent them away from his son Isaac eastward, to the land of the East. [Gen. 25:6]

Abraham married Sarah and had one son. He married Keturah and had six sons. He also had one son by Hagar, and the Ramban tells us he married her:

Abraham did not plan on building a family with Hagar rather than Sarah. His only intent was to do Sarah's will so she would be pleased… Sarah wanted Hagar to also be Abraham’s wife, so he would have a child with a wife and not just a concubine.

So who are these "concubines" (plural) and who are their "sons"?

Answer 1

Interestingly, Radak believes the passage does not refer to any of Abraham's 8 sons, but to the sons of the concubines of his male servants:

He had no concubines from whom he had fathered children, but the concubines in his household were concubines of male servants in his household. Their children were, of course, members of Avraham’s household who had been of service to Avraham and who had adopted his lifestyle. He gave them gifts consisting of silver and gold and livestock according to his own evaluation of who deserved how much. He most certainly gave even more valuable gifts to Ishmael and to the sons of Keturah even though the Torah does not specifically mentions this. If he gave gifts to people towards whom he had no legal obligation at all, it is obvious that people towards whom he did have such an obligation would receive their due.

Answer 2

I think it's just a product of the grammar and shouldn't be read to deep to determine the numbers of concubines.

In English, we tend to say "sons of bitches" rather than "sons of a bitch"

You could try to say that Ishmael was also sent to the East, but Torah says he lived in the wilderness of Paran, which is to the South and one of the stops of the Jews in the desert.