At the root of all of the Jewish people’s souls, there is a level at which
they are all in fact a single, unified soul, known as kenesset Yisrael, the
congregation of Israel.
This comprehensive soul, incorporating all the souls of the Jewish people, is divided into 600,000 individual souls. Although the census of Jews throughout history often yielded more than 600,000, this number is the root and foundational number comprising the Jewish people, defining and characterizing them.
Any number less than this no longer constitutes the Jewish nation.
Rabbi Adin Steinsaltz