I thought quoting the passage might be a help at this point:
Galatians 3:16 (ESV)
Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” referring to many, but referring to one, “And to your offspring,” who is Christ.
Now since the ESV is less interpretative in it’s translations than the NIV I am assuming is also more accurate.
At no point in Genesis is word “offsprings” used; it is always “offspring”. The word is always used in the sense of more than one, and often in terms of being a multitude.
However, since Paul is thoroughly expert in the OT we must assume he knows what he is talking about.
Putting the quote in a wider context, we see:
Galatians 3:15-19 (ESV)
To give a human example, brothers: even with a man-made covenant, no one annuls it or adds to it once it has been ratified. [16] Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” referring to many, but referring to one, “And to your offspring,” who is Christ. [17] This is what I mean: the law, which came 430 years afterward, does not annul a covenant previously ratified by God, so as to make the promise void. [18] For if the inheritance comes by the law, it no longer comes by promise; but God gave it to Abraham by a promise.
[19] Why then the law? It was added because of transgressions, until the offspring should come to whom the promise had been made, and it was put in place through angels by an intermediary.
Perhaps it is to be understood in the same way we understand Genesis 3:15:
Genesis 3:15 (ESV)
I will put enmity between you and the woman,
and between your offspring and her offspring;
he shall bruise your head,
and you shall bruise his heel.”
The fact that a word that has the meaning of “ a number of” is here being used to refer to one only indicates that it could be used is such a way, and we need to look a more widely to understand it.
Genesis 12:2-3 (ESV)
And I will make of you a great nation, and I will bless you and make your name great, so that you will be a blessing. [3] I will bless those who bless you, and him who dishonors you I will curse, and in you all the families of the earth shall be blessed.”
We understand this verse to be referring to Christ. Paul is telling us that the promise to Abraham into this context. This does not, however, explain his wording “And to offsprings”, since it is never used.