Why there are so few Jews (14 million) if we were promised to be multiplied as "stars of heavens" (Gen 22:17, Gen 26:4 e.t.c) which numbers in billions of billions?
The answers are in the Written Torah of'course.
First of all, Jews (Hebrews to be more specific, as Judah is just one tribe) are not numerous people:
YLT Dt 7:7 'Not because of your being more numerous than any of the peoples hath Jehovah delighted in you, and fixeth on you, for ye are the least of all the peoples,
YLT Dt 7:8 but because of Jehovah's loving you, and because of His keeping the oath which He hath sworn to your fathers, hath Jehovah brought you out by a strong hand, and doth ransom you from a house of servants, from the hand of Pharaoh king of Egypt.
Torah calls Hebrews "the least of all the peoples".
Secondly, Jews do not follow Written Torah. They follow their traditions and so-called Oral Torah. This is why Written Torah says the following:
YLT Dt 4:27 and Jehovah hath scattered you among the peoples, and ye have been left few in number among the nations, whither Jehovah leadeth you,
YLT Dt 28:62 and ye have been left with few men, instead of which ye have been as stars of the heavens for multitude, because thou hast not hearkened to the voice of Jehovah thy God.
As you can see, this questions is answered directly in the Torah. Torah says more:
YLT Dt 9:4 'Thou dost not speak in thy heart (in Jehovah thy God's driving them away from before thee), saying, For my righteousness hath Jehovah brought me in to possess this land, seeing for the wickedness of these nations is Jehovah dispossessing them from thy presence;
YLT Dt 9:5 not for thy righteousness, and for the uprightness of thy heart, art thou going in to possess their land; but for the wickedness of these nations is Jehovah thy God dispossessing them from before thee; and in order to establish the word which Jehovah hath sworn to thy fathers, to Abraham, to Isaac, and to Jacob;
Even under Moses, Hebrews rebelled and disobeyed the commandments, so the only reason why God did not destroy them was because He gave an oath to our forefather Abraham.
YLT Lv 26:43 'And--the land is left of them, and doth enjoy its sabbaths, in the desolation without them, and they accept the punishment of their iniquity, because, even because, against My judgments they have kicked, and My statutes hath their soul loathed,
YLT Lv 26:44 and also even this, in their being in the land of their enemies, I have not rejected them, nor have I loathed them, to consume them, to break My covenant with them; for I am Jehovah their God;--I hope this answers this question clearly.