One of the most common but erroneous claims made concerning Galatians 3:16 goes like this: Every time the word "seed" is used in Genesis 12-22, it is only referring to Jesus.  

Paul in Galatians 3:16 is not saying that every time Genesis talks about Abraham’s “seed,” it only means Jesus. The Hebrew word zeraʿ (“seed”) can mean one person or many. In Genesis, it clearly refers to both:

Plural/collective:

“Count the stars… so shall your descendants be.” (Genesis 15:5)

“I will make your descendants as the dust of the earth.” (Genesis 13:16)

Singular/representative:

“…through Isaac your descendants shall be named.” (Genesis 21:12)

“…in your seed all the nations of the earth shall be blessed.” (Genesis 22:18)

Genesis itself shows the promise narrowing to one chosen heir within the many, first it narrowed to Isaac, then it narrowed to Jacob, and ultimately it narrows to the Messiah.

Paul’s point is that Christ is the ultimate “Seed” through whom the promise comes, and those who belong to Him are also counted as Abraham’s seed (Galatians 3:29). He’s not rewriting the Old Testament; he’s following its own pattern: many descendants, one promised heir. - Joel Richardson