Mathematics

Rev 5:11
11   Then I looked and heard the voice of many angels,
numbering thousands upon thousands,
and ten thousand times ten thousand.
They encircled the throne and the living creatures and the elders. (NIV)

If one third or 200 million angel fell with satan that would be 66,666,666...
an interesting number.
Of course no one knows how many angels are in heaven,
and I am in no way suggesting that there were
two hundred million angels in heaven when satan fell.
I'm pointing out that once we understand the possible population of heaven,
we can understand the size of satan's army.
For every two million angels that were in heaven when satan fell,
66,666 fell with him...
with the exception of the very worst that are locked in the abyss,
these millions of fallen angels are in the air about us doing satan's bidding.
God chose his elect to fill the void in heaven left by satan and the fallen angels.