The word "seed" (Hebrew "zera") is figuratively used to mean "posterity or descendants" and literally mean a "child". The meaning will depend on the context of the message as from the verses below.
Gen 3:14 And the LORD God (Hebrew "Elohiym") said unto the serpent, Because thou hast done this, thou art cursed (Hebrew "Arar" or "Alah") above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life:
Gen 3:15 And I will put enmity between thee and the woman, and between thy seed (posterity/offspring) and her seed (posterity/ offspring); it shall bruise thy head, and thou shalt bruise his heel.
Gen 4:25 And Adam knew his wife again; and she bore a son, and called his name Seth: For God, said she, hath appointed me another seed (child) instead of Abel, whom Cain slew.
Mark 12:20 Now there were seven brethren: and the first took a wife, and dying left no seed (posterity/offspring).
I will quote from 3 witnesses in order to comply with the Mosaic law for 2 or 3 witnesses for establishing the matter. Deutronomy 17:6 At the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death; but at the mouth of one witness he shall not be put to death. Matthew 18:16 Jesus said: But if he will not hear thee, then take with thee one or two more, that in the mouth of two or three witnesses every word may be established.
1) Based on Yahweh's own words
Gen 21:12 And God said unto Abraham, Let it not be grievous in thy sight because of the lad, and because of thy bondwoman; in all that Sarah hath said unto thee, hearken unto her voice; for in Isaac shall thy seed (posterity) BE CALLED (Hebrew "qara" for called or CHOSEN).
Mat 20:16 (Jesus said) So the last shall be first, and the first last: for many be called, but few CHOSEN.
Gen 21:13 And also of the son of the bondwoman will I make a nation (Hebrew "Goy" for foreign nation - hence, Gentile nation), because he is thy seed (posterity).
Gen 26:24 And the LORD appeared unto him(Isaac) the same night, and said, I am the God of Abraham thy father: fear not, for I am with thee, and will bless thee, and multiply thy seed (posterity) for my servant Abraham's sake.
Gen 28:13 And, behold, the LORD stood above it, and said (to Isaac's son Jacob), I am the LORD God of Abraham thy father, and the God of Isaac: the land whereon thou liest, to thee will I give it, and to thy seed (posterity)
Gen 32:28 And he said, Thy name shall be called no more Jacob, but ISRAEL: for as a prince hast thou power with God and with men, and hast prevailed.
Gen 35:10 And God said unto him, Thy name is Jacob: thy name shall not be called any more Jacob, but Israel shall be thy name: and he called his name Israel.
Gen 35:12 And the land which I gave Abraham and Isaac, to thee I will give it, and to thy seed after thee will I give the land.
2) Based on St. Luke's record
Act 13:22 And when he had removed him, he raised up unto them David to be their king; to whom also he gave testimony, and said, I have found David the son of Jesse, a man after mine own heart, which shall fulfil all my will.
Act 13:23 Of this man's seed hath God according to his promise raised unto Israel a Savior, Jesus
3) Based on the elaboration from St. Paul - a Jew and Pharisee
Rom 9:6 Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel:
Rom 9:7 Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called (chosen).
Rom 9:8 That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.
Rom 9:9 For this is the word of promise, At this time will I come, and Sarah shall have a son.
Gal 3:16 Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
4) Based on St. John's record
John 8:31 Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed;
John 8:32 And ye shall know the truth, and the truth shall make you free.
John 8:33 They (Jews) answered him, We be Abraham's seed, and were never in bondage to any man: how sayest thou, Ye shall be made free?
John 8:34 Jesus answered them, Verily, verily, I say unto you, Whosoever committeth sin is the servant of sin.
John 7:42 Hath not the Scripture said, That Christ cometh of the seed (posterity/offspring) of David, and out of the town of Bethlehem, where David was?
To sum up, both Ishmael and Isaac are the seeds of Abraham (Gen 21:12-13) but only one of them was chosen or be called. The seed will fulfil the salvation plan of God. That true seed is Jesus Christ who will establish an eternal Kingdom in new Jerusalem City. It is apparent that Yahweh's chosen seed is through the bloodline of Isaac as the covenant was repeated with Isaac (Gen 26:24) and then with his son Jacob (Gen 28:13). There is no single verse in the Bible about Yahweh appeared and said to Ishmael except the chosen seed Isaac (Gen 26:24). Another evidence is that the nation of Israel exists even today while the name of Ishmael is not a nation in the world.
Whosoever disbelieve about the promised posterity of Isaac is because they have been beguiled by the devils (Sura 19:83 and Sura 59:16). May God open the eyes of understanding of the blind in Jesus' Name, the everlasting seed of Abraham.