Paul now turned to the objection that when God gave the Law He terminated justification by faith alone. He reminded his readers, with a human analogy, that even wills and contracts made between human beings remained in force until the fulfillment of their terms. Likewise the covenant God made with Abraham remains in force until God fulfills it completely. The promises made to Abraham extended to his descendants as well as to him personally. They even extend to Christ, the descendant of Abraham who became the greatest source of blessing God promised would come through Abraham"s descendants. Paul did not mean that Christ fulfilled the Abrahamic Covenant completely. He meant that through Christ, the descendant of Abraham, God continued to fulfill the Abrahamic Covenant. The Mosaic Law did not supersede (take the place of) the Abrahamic Covenant. [source][source][source]
The Hebrew word for "seed" or "offspring" (zera, Galatians 3:16) is a collective singular that can refer either to one descendant or many descendants. An English collective singular, for example, is "sheep" that can refer to one sheep or many sheep. Both "seed" and "offspring" are also collective singulars in English. Paul explained that the seed God had in mind in Genesis 13:15; Genesis 17:8 was the one descendant, Christ. [1][source]
"The term seed not uncommonly denotes all the descendants of some great ancestor, but it is not normally used of one person. Used in this way it points to the person as in some way outstanding; the seed is not simply one descendant among many but THE descendant." [2][source]
The Four Seeds of Abraham in ScriptureNatural SeedAll physical descendants of Abraham Genesis 12:1-3; Genesis 12:7; et al. Natural-Spiritual SeedBelieving physical descendants of Abraham Isaiah 41:8; Romans 9:6; Romans 9:8; Galatians 6:16Spiritual SeedBelieving non-physical descendants of Abraham Galatians 3:6-9; Galatians 3:29Ultimate SeedJesus Christ Galatians 3:16; Hebrews 2:16-17[source]