We will never know exactly how many women disguised themselves as men and fought in the Civil War. At the close of the hostilities, it was estimated that approximately 400 women had managed to enlist, but this number is almost certainly too low.
I write narrative nonfiction, creating lively scenes through action and the use of quotes from firsthand accounts, all based on rigorous research. If I say a character leaned against a fence on a windy day, than I have at least two sources to back up these details.