Good question. Before I address your question of the promise of the very great number of descendants promised Abraham, I’d like to put your question in proper context.
The greatness of the Jewish People as a nation descended from Abraham is definitely not because of their great population. The Torah states, “God did not love you nor choose you because you were more in number than other nations – for you are the smallest of them all” (Deut. 7:7).
The commentaries explain that the Jewish People must be counted according to the immeasurable impact that they have made in the world and not according to their physical numbers. There is no doubt that the Jews have impacted the world in a way that is incomparable to any other nation in the world. This is so, regardless whether this impact is measured in religious importance, medical advances or other noted achievements throughout the world.
Regarding your question about comparing their number to the countless dust of the earth and stars in the sky, the commentaries explain this in various manners.
For example, Rabbi S. R. Hirsch explains that it is referring to the vast number of descendants of one set of parents – Abraham and Sarah – who have spanned throughout history in a continuous chain. This chain of the Jewish People is both physical and spiritual in nature – for example, converts to Judaism are called the “son and daughter of Abraham and Sarah” according to Jewish Law. And this chain is eternal.
It is interesting to note, as Rabbi Hirsch writes, that God’s promise to Abraham and Sarah was when they had passed their childbearing years and Abraham thought they would remain childless. God told Abraham to go outside look at the great number of stars in the heaven. The underlying message was, “Look! Just as I’ve directly created all these stars in this ‘other world’ above yours, I can and will change the normal order for you and directly create for you an endless chain of uncountable descendants!”
