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 25:1-2
Abraham had two sons; one by a bonds-maid, the other by a free woman. - Galatians 4:22
By faith, Abraham when he was tried offered up Isaac... his only begotten son. - Hebrews 11:17
So how many kids for Abraham (per Genesis), 2 (per Galatians) or 1 (per Hebrews)?
Galatians isn't hard to deal with, since it isn't saying that Abrahamonly two kids -- it's just picking two out of the lot (the two most important for the context) and using them as examples. Only our modern concern with side details requires the added thought, "He had six others by another women, but that doesn't come into this story."
That leaves Hebrews (which should not mention the other 6, which were born after the time described) -- did the writer of Hebrews forget Ishmael?
Hebrews is here making use of the LXX version of Genesis 22:2, with one exception: where Hebrews says "only begotten" the LXX says "beloved." On the other hand, a variant tradition of the LXX, used by Aquila and Josephus, also says "only begotten."
Why? Both words derive from the Hebrew yahid, which can carry both meanings. This is the sense in which "only begotten" should be understood. It is therefore not a statement of birth order or number but of prominence.