This achievement is a little confusing as a game doesn't specify what a "stash" is, and there is no tracker either.
As far as I can tell, a stash is location that soldiers had previously used as a hideout and left supplies behind, or a red crate/box that was left behind randomly with supplies inside. Some fallen soldiers also have supplies on them, and there is also military-grade ammo locations, as well as a safe in the level although I do not think they count towards this achievement. However, it is advised that you make sure to get 100% of what I listed above as the game does not specify what counts and what does not.
I have made a video showing everything that you can pick up in the level, which is about 50 total items. If you follow along with the video, you will unlock the achievement by the end.