When God told Abraham to kill his son Isaac, God referred to Isaac as Abe's "only son."
Take now thy son, thine only son Isaac ... and offer him there for a burnt offering. Genesis 22:2
1600 or so years after Abraham supposedly lived, the author of Hebrews repeated God's reference to Isaac as Abraham's "only son."
By faith Abraham when he was tried, offered up Isaac ... his only begotten son. Hebrews 11:17
Abraham was 86 years old when his son Ishmael was born.
And Hagar bare Abraham a son: and Abram called his son's name, which Hagar bare, Ishmael. And Abram was fourscore and six years old, when Hagar bare Ishmael to Abram. Genesis 16:15-16
Sixteen years later, when Abraham was 100 years old, Isaac was born.
For Sarah conceived, and bare Abraham a son is his old age .... And Abraham called him Isaac. ... And Abraham was an hundred years old, when his son Isaac was born. Genesis 21:2-5
Abraham had two sons; the one [Ishmael] by a bond-woman [Hagar], and the other [Isaac] by a free woman [Sarah]. Galatians 4:22
(Depending on whether or not Ishmael is Abraham's son.)
Many years later, Abraham "took" another wife (or concubine) and had six more sons.
Then again Abraham took a wife, and her name was Keturah. And she bare him Zimran, and Jokshan, and Medan, and Midian, and Ishbak, and Shuah. Genesis 25:1-2Now the sons of Keturah, Abraham's concubine: she bare Zimran, and Jokshan, and Medan, and Midian, and Ishbak, and Shuah. 1 Chronicles 1:32
