So, I don’t think there is a way to nail the question asked in the OP down except in broad terms. I did some searching and it looks to me as if the arms industry in the US employs over 2 million workers and does about $395 billion in sales annually. A hit of over $100 billion would be pretty large…assuming that if we didn’t sell stuff to the Saudi’s it wouldn’t be sold at all, which I tend to doubt. Some US systems (those we can actually sell) are in high demand, others…not so much. I haven’t looked into what the Saudi’s are buying, but unless it’s air defense missiles (which I doubt) mainly, I’d say that if they don’t buy them someone else will. If it IS mainly air defense missiles then that could be more problematic. Frankly, our air defense missiles aren’t on par with what several other countries make and there isn’t a huge demand for the things, since they also tend to cost as much or more than better performing systems.
Anyway, I’d go with a WAG of less than 100k workers, just based on straight cost verse number of laborers, but I’d say (another WAG) that it would be a lot less than that…I don’t know, maybe a few 10’s of thousands?
Of course, I don’t think it really IS $100+ billion dollar sale that the Saudi’s are talking about doing, so I think that the actual impact of this would be minimal…in the short term. Long term, it would be a bigger problem, since the Saudi’s are a major US arms buyer, which means parts and systems as well as the capital purchases, but I don’t think that those are in any sort of long term danger unless the Saudi government folds at some point and the new regime is like Iran, having a bunch of US weapons and systems but who is officially banned from parts, maintenance or new systems.