In all, Abraham had eight sons.
Abraham’s first son was Ishmael through Hagar, his wife’s Egyptian maid (Genesis 16:1–4).
Abraham’s second son was Isaac through Sarah, his wife (Genesis 21:1–3). Isaac was the son God had promised Abraham (Genesis 15:4–5).
After Sarah died, Abraham had six sons through Keturah, another concubine: Zimran, Jokshan, Medan, Midian, Ishbak, and Shuah (Genesis 25:1,6). Keturah’s sons became the fathers of Arabian tribes living east of Israel.
Some people claim that the Bible makes an error in regards to the number of Abraham’s sons. In Genesis 22:2, God speaks to Abraham after the birth of Ishmael, referring to Isaac as “your son, your only son, whom you love.” Then Hebrews 11:17 identifies Isaac as Abraham’s “one and only son.” And Galatians 4:22 mentions only Isaac and Ishmael: “It is written that Abraham had two sons.” How could Abraham be said to have an “only son” and “two sons,” when in reality he had eight sons?
There is no true contradiction in the above passages. Isaac was the only son who was promised to Abraham and through whom Abraham would become the father of many nations (Genesis 12:1-3; 17:1-8; 21:120). Also, Isaac was the only son of Sarah and Abraham—Sarah being specifically mentioned in the prophecies of Genesis 17:16-21 and 18:10. In addition, Isaac is the only son born in an official marriage: Hagar and Keturah were both concubines. While God blessed the concubines’ sons for Abraham’s sake, those sons had no part in the inheritance. Isaac was the one and only rightful heir to the promise (Genesis 15:4-5, 25:5).
Genesis 22:2 and Hebrews 11:17 both refer to Isaac as Abraham’s “only son” because those passages concern God’s promise and covenant. Since Abraham’s other seven sons are not part of the covenant, they are irrelevant to the issue and not mentioned as sons. Abraham had other sons, but only one son of promise.
Paul refers to Abraham’s two sons in Galatians. Letters of the Bible are is like listening to one half of a phone conversation. So, we have to infer what has been said or the issues with those who the letter is directed towards. Paul is writing to a group of people with specific situations with a specific set of pastoral concerns. So, we must understand what the intention was of the original author before we can apply what is being said to our day, our life. For a study on Galatians, to the ‘Bible Study Series’ tab and click on Galatians. There are six videos, one video per chapter.
Spiritually speaking, Abraham has many, many sons. The Bible points to the faith of Abraham (Genesis 15:6) and states that “those who have faith are children of Abraham” (Galatians 3:7; cf. verse 9). Those who exercise the same faith that Abraham had are showing themselves to be like him, spiritually, and so can be rightly called his “children.” All who trust in Christ, as Zacchaeus did, become true sons of Abraham (Luke 19:9). “The promise comes by faith, so that it may be by grace and may be guaranteed to all Abraham’s offspring . . . to those who have the faith of Abraham. He is the father of us all” (Romans 4:16).