I have a character, he's supposed to lose an arm. Would nerve damage caused by something sharp and pointy suffice? Would it render his arm useless immediately, if not how soon? What hits first - pain or the lack of response in his arm? He's held down, possibly trying to fight back and or get up. There's also a heel on his shoulder/arm. How long would the pain last in that case? He's got no way of getting immediate treatment, how long till the bleeding wears him down?
So this is an interesting question, in that it sounds like you want to produce a character with a “dead arm”, without actually severing it. I like the creativity!
So here’s WebMD’s drawing of arm nerves looks like this:
( source )
But you see how they both sort of tuck together right in the armpit, riiight under the shoulder, just distal (away from the body) to the humeral head (the top of the arm bone)? A bad guy with a strong knowledge of A&P could put a stab wound right there, and sever all the motor nerves in that arm forever. It may also nick the bone, and/or the brachial artery.
I would imagine pain would come first, then panic when the arm doesn’t move, then eventually the realization that his arm is dead. And this sounds like an injury that would hurt for a while. Hope that’s useful!