How many angels left with Lucifer when God banished them from heaven?
There is a passage in Revelation, chapter 12, verses 3 and 4, which may be the only passage to tell us the proportion of the angelic host to have fallen from their exalted state in heaven:
"Then another sign appeared in heaven: and behold, a great red dragon having seven heads and ten horns, and on his heads were seven diadems. And his tail swept away a third of the starts of heaven and threw them to the earth. And the dragon stood before the woman who was about to give, birth, so that when she gave birth he might devour her child."
There are other verses which speak of angels and a war in heaven, the result of which was their being cast out of heaven along with their leader, Satan (cf. vv. 8-9; 9:1; Dan. 8:10; Jude 6; 2 Pet. 2:4). Moreover, other passages link metaphorically the "stars of heaven" (or simply "stars") with angelic beings (see Job 38:6,7; and Isa.14:12,13).
To attempt to come up with an even approximate number of angels who kept their abode in heaven is an exercise in futility. True, Revelation 5:11 says there are 10,000 times 10,000 angels--besides living creatures and elders--around God's throne, though some versions (e.g., the NIV) describe the number of angels as "myriads of myriads and thousands of thousands." Then thousand times ten thousand is a hundred million, but then thousands of thousands on top of the hundred million will still not give us an accurate count.
About all we can say about the number of angels is that there are a lot of them. Who knows, there could (I repeat, could) be as many angels as there are stars, and the number of stars . . ., well, lets put it this way: only God Himself knows not only how many there are, but He also has a name for each one (see Psalm 147:4; cf. Gen.1:16, where it says simply, "He also made the stars," as if it was no big deal!).