Since it is
the nation's crime it is uncountable therefore use
amount.To use number you would have to change the sentence to:
A substantial number of the nation's crimes are committed ..
However, since we have the word number used in the sentence it would not sound good to have it repeated.
A substantial amount of the nation's crime is committed by a small number of bad actors --
A substantial number of the nation's crimes are committed by a small number of bad actors --
The first sentence sounds much better.