Adam Clarke on Genesis 15:5

Look now toward heaven - It appears that this whole transaction took place in the evening; see on Gen 13:14 (note). Abram had either two visions, that recorded in Gen 15:1, and that in Gen 15:12, etc.; or what is mentioned in the beginning of this chapter is a part of the occurrences which took place after the sacrifice mentioned Gen 15:9, etc.: but it is more likely that there was a vision of that kind already described, and afterwards a second, in which he received the revelation mentioned Gen 15:13-16. After the first vision he is brought forth abroad to see if he can number the stars; and as he finds this impossible, he is assured that as they are to him innumerable, so shall his posterity be; and that all should spring from one who should proceed from his own bowels - one who should be his own legitimate child.