Romans 9:27-28 Isaiah cries out concerning Israel, “Though the number of B’nei-Israel be as the sand of the sea, only the remnant shall be saved. For Adonai will carry out His word upon the earth, bringing it to an end and finishing quickly.”
Paul is quoting Isaiah 10:22-23 For though your people, O Israel, be as the sand of the sea, only a remnant of them will return. Destruction has been decreed. Justice overflows. For a complete destruction, as decreed, will Adonai Elohei-Tzva’ot make throughout the whole land.
Paul is limited (by space and time) in his letter to the Romans. He can’t quote all of Scripture. Neither can he make every argument or explanation. Read on here