Everyone knows that the more common people, of the First Order, either live lives of luxury or tyranny, and the troopers are trained at a young age and are practically expendable.
But what do they do in the cases when they're injured enough to where they can still live another day?
It would appear that they have ready access to Medical Droids and Bacta treatment aboard the Supremacy.
It seems likely that injured soldiers would also be given treatment and then returned to their units.
Note that Finn's first instinct in the event of his comrade being shot is to render medical assistance and first-aid, so clearly Stormtroopers are valued beyond their immediate capacity to carry a gun and follow orders.
As part of the last squad to arrive on the scene, FN-2187 hadn’t even triggered his sidearm when FN-2003 crumpled to the ground, his armor smoking. FN-2187 dropped down beside his buddy immediately. He wanted to render some kind of medical aid, but the wound was too severe. The trooper they all called Slip was beyond help.
