Gen 22:17 That in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heaven, and as the sand which is upon the sea shore;
God promised Abraham that his descendants would be as the sand of the sea, and as the stars of heaven. These two metaphors are in direct apposition to each other, and explain each other. The former metaphor is more plentiful in the Bible, so let's take a look at it:
Psa 139:18 If I should count [God's thoughts for me], they are more in number than the sand: when I awake, I am still with thee.
Psa 78:27 He rained [birds that they could eat] also upon them as dust, and feathered fowls like as the sand of the sea
Job 29:18 Then I said, I shall die in my nest, and I shall multiply my days as the sand.
1Ki 4:20 Judah and Israel were many [under Solomon's reign], as the sand which is by the sea in multitude, eating and drinking, and making merry.
1Ki 4:29 And God gave Solomon wisdom and understanding exceeding much, and largeness of heart, even as the sand that is on the sea shore.
2Sa 17:11 Therefore I counsel that all Israel be generally gathered unto thee, from Dan even to Beersheba, as the sand that is by the sea for multitude; and that thou go to battle in thine own person.
1Sa 13:5 And the Philistines gathered themselves together to fight with Israel, thirty thousand chariots, and six thousand horsemen, and people as the sand which is on the sea shore in multitude: and they came up, and pitched in Michmash, eastward from Bethaven.
Jdg 7:12 And the Midianites and the Amalekites and all the children of the east lay along in the valley like grasshoppers for multitude; and their camels were without number, as the sand by the sea side for multitude.
Jos 11:4 And they went out, they and all their hosts with them, much people, even as the sand that is upon the sea shore in multitude, with horses and chariots very many.
Deu 28:62 And ye shall be left few in number, whereas ye were as the stars of heaven for multitude; because thou wouldest not obey the voice of the LORD thy God.
If you actually read all of those, color me impressed. The point being: Several armies and peoples are compared in number to the sand of the sea. As are birds, camels, days, and thoughts. "As the sand of the sea", or "as the stars of heaven" are not to be taken literally - they are simply idiomatic expressions meaning "very, very many". The same way I might say, "I'm hungry enough to eat a horse", or "enough of something to fill the Grand Canyon". God promised to make Abraham's descendants very, very numerous. 600 000 men over 20 (that is, probably ca. 2 million people) may or may not be counted as "very, very many" - 2 mill would be more than Job's days (6000 years are about 2 million days), but less than the number of Jews in Solomon's days. And under Solomon's reign, it specifically says (twice) that the Jews were as the sand of the sea - and by implication also as the stars of heaven. Moreover, Deut 28:62 seems to speak as the promise fulfilled while the Jews still had only 600 000 fighting men.