Here we have a Paul's trick in his use of the Old Testament:
The promises were spoken to Abraham and to his seed. Scripture does not say "and to seeds," meaning many people, but "and to your seed," meaning one person, who is Christ. (Galatians 3:16)
Note that Paul is arguing that "sperma" (seed) can't be understood as a collective because it is a singular noun. However, this argument can easily be refuted by reading how "sperma" was used in Genesis 13:15-16.