Genesis 15:5

And He brought him forth without, and said, Look up now to the heavens, and number the stars, if thou art able to number them: and he said, So will be thy sons.

AND HE BROUGHT HIM FORTH OUTSIDE. According to the simple interpretation of Scripture it means that He brought him forth from his tent into the open so that he could see the stars. And according to the Midrash its explanation is as follows: G-d said to him, “Leave your astrological speculations, for you have seen by the constellations that you are not destined to raise a son. “Abram” indeed may not have a son, but “Abraham” will have a son; “Sarai” will not bear a child, but “Sarah” will bear a child. Thus the language of Rashi.
But ‘Abram’ did have Ishmael!
The meaning of the Midrash, however, is that Abram sought a son who would qualify to become his heir, [which excluded Ishmael, who would not inherit him], even as he said, And, lo, my household slave will be mine heir. The Holy One, blessed be He, then said to him, “This man shall not be thine heir, but one born of thine own body shall be thine heir, and leave your astrological speculations. ‘Abram’ will not have a son as his heir, but ‘Abraham’ will have a son as his heir.”
It is also possible that the astrological speculation concerned the pair together, namely, the “Abram and Sarai” as a pair will not beget children, and the Eternal now assured him that “Abraham and Sarah” will beget children. In my opinion, however, [the Divine assurance now given to Abraham did not mention Sarah, rather], her name is an addition on the part of the Midrash, meaning that such indeed was the case also with Sarah. The Holy One, blessed be He, however, did not assure him concerning Sarah at the present time. Even at the time of the prophecy concerning the circumcision, Abraham was still in doubt whether “Sarah” would bear a child.

ויוצא אותו החוצה, he had received the prophetic vision and words of G’d while in the tent. As part of this prophetic insight, Avram felt that G’d took him outside the tent, showing him the stars. This was the מחזה, “the vision.” We have a parallel situation in Ezekiel 8,3 ותבא אתי ירושלמה במראות אלוקים, “He brought me to Jerusalem in visions of G’d;” [the prophet never set foot in Jerusalem, but lived in Babylonia all his life. Ed.] The new element now was that whereas previously G’d had compared Avram’s offspring as being “like the dust of the earth,” now it is described as “as numerous as the stars in heaven.” Both statements, of course, have to be understood as exaggerations, as we pointed out already on 13,16. We also mentioned the allegorical explanations offered on that verse. It is possible, that in messianic times the Jewish people will be comparable to the stars in that just as no one is able to impose his will on the stars in heaven, so no one will be able to impose his will on the Jewish people. They will shine on earth without interruption just as the stars shine in the sky without dimming. In Bereshit Rabbah 44,10 the words ויוצא אותו are understood as G’d telling Avram to free himself from his astrological speculations according to which he was not able to beget children. Once his name would be changed to Avraham, there would be no reason why a person by that name could not beget children. As soon as G’d would change his name he would be able to sire children, something that proved correct.

כה יהיה זרעך, “so shall be your seed.” Rabbi Tanchuma son of Acha, basing himself on this phrase, commented that it is an assurance that there will never be fewer than 30 righteous people on earth at all times. He uses the word יהיה, “will be (singular),” as his proof. [The numerical value of the combined letters in that word is 30. Ed.]