[–] Diogenes_The_Cynic 0 points 1 point 1 point (+1|-0) ago
The best evidence is that the army themselves kept records of the fighting.
The evidence against it is from anthropologists that openly admit there is no way to determine how many Native Americans were alive at any point, so any numbers are speculation.
[–] TheBuddha 0 points 1 point 1 point (+1|-0) ago
Here's a well sourced bit of work to draw your own conclusions from:
http://americanhistory.oxfordre.com/view/10.1093/acrefore/9780199329175.001.0001/acrefore-9780199329175-e-3