Why did Moshe compare Yisrael to the numberless stars, when their numbers were known to be six hundred thousand?
Rashi: What Moshe meant was that Yisrael's existence is eternal, like ha'Yom (the sun during the day, 1 and the moon and stars at night). 2
Because "ha'Yom" means 'ke'ha'Yom' - like the sun and moon (Sifsei Chachamim).
And the word la'Rov" refers, either to a future date, when they will become numerous like the stars (See Sifsei Chachamim) or, not quantitively, but qualitively - See Oznayim la'Torah.