Revelation 9:16 - Ellicott's Commentary On The Whole Bible

And the number... — Translate, And the number of the armies of the cavalry was two myriads of myriads. I heard the number of them. The writer heard, perhaps from some herald angel, the number of this vast army of horsemen; it was twice ten-thousand times ten thousand — i.e., two hundred millions. The number is like an echo from Psalms 68:17 — “The chariots of God are twenty thousand (two myriads), even thousands (or, thousands of thousands) of angels.” This utterly bewildering number might have been sufficient to keep interpreters from looking for some slavishly literal fulfilment: it simply stands for an immense host, and may serve to point out the prolific powers of retribution — the harvest of sin is misery, multiplied thirty, sixty, one hundred-fold.