וְשַׂמְתִּ֥י אֶֽת־זַרְעֲךָ֖ כַּעֲפַ֣ר הָאָ֑רֶץ אֲשֶׁ֣ר ׀ אִם־יוּכַ֣ל אִ֗ישׁ לִמְנוֹת֙ אֶת־עֲפַ֣ר הָאָ֔רֶץ גַּֽם־זַרְעֲךָ֖ יִמָּנֶֽה׃
And I will make thy seed as the dust of the earth; so that if a man can number the dust of the earth, then shall thy seed also be numbered.
Read kabbalistic (mystical) interpretation of Genesis 13:16: and classical mystical sources.