Revelation12:3-4 (KJV)

And there appeared another wonder in heaven; and behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads.

And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born

Stars are generally thought to refer to angels and the great dragon is Lucifer or Satan as he is now called.  So, one third of all angels fell. A very large number-billions and maybe trillions?