"astounding amount" is correct and usable in written English.
It is used to refer to an unbelievable, or amazing amount. For example, "I won an astounding amount of money in the lottery!".
The historian offers an astounding amount of detailed reports and facts but with no unifying concept.
He still has an astounding amount of hair for someone his size.
I had managed to pack an astounding amount of snow up my nose.
So what we are seeing is an astounding amount of concentration.
This series of Big Brother has received an astounding amount of criticism.
Barcelona's mesmerizing playing style, this WSJ.com article reveals, involves an astounding amount of running on its part.
They hid astounding amounts of cash inside walls.
The TSA seems like a caricature of a wasteful bureaucracy, spending astounding amounts of money even during a sluggish economic recovery.
From age 15 to 27, Phelps has been one of the best — or the best — in the world in a sport that takes astounding amounts of time and effort.
And yet so many other computational systems that exhibit astounding amounts of problem-solving capabilities seem to draw very little inspiration from nervous systems—Wolfram Alpha, for example, or Boston Dynamics' robots, or a lot of the expert systems AI work done a few decades ago.
These powerful fountains of energy, emitting astounding amounts of x-rays, visible light, and radio waves, are thought to be powered by monstrous black holes in the cores of galaxies at the edge of the observable universe.
