So many factors;
Resources and such. The cool thing about infrastructure is, it is labor and resource intensive in the short term, but once in place it takes relativly few to run it and maintain it. <proper maintennce is important though.>
The raw materials, turning into materials that then can be turned into a finish product. That is where alot of the population will be used. The whole chain of events that takes the ore from the ground, transports it, processes it <and gathering the resources to process it, coal, coke, wood etc> and then to turn it into iron and from there into the actual product to be used. That is where the numbers are needed.
And of course having all of the assets within your region is key. But, once those are in place it takes realtivly few miners to pull it from the ground.
And then, few to man the machinery, fewer if its automated. And automatiion can come not just woth modern electrical power, but also water power. If you have a water way localy, you can make a mill to grind your ore for processing making it easier to turn into metal.
And you can as is mentioned in many of the adventures, mom and pop production in garage workshops. That is the cool thing, we have the technology that is advanced to do things quicker and easier, and we have the knowledge that is advanced so we do not have to relearn it.
Thus, a small machineshup in the size of someones two car garage could turn out a decent production capacity with say, a hydraulic press <a modified press from Harbor Freind Tools> set it up with the propper dies and you are in buisness.
I could see for instance a machineshop turning out a couple dozen AR-18s a day, and thus in my T2K world in the US AR-18s are quite common as are MAC-11s and 10s, since both use mostly stamped parts in their construction.
I can see a society depending on the resources and talent available returning to at least WWII level of technology in short order if they are well organized and have the assets available to them, as well as being left unmolested.
Of course first things being first they would need to provide the basics, those being food and shelter. Those would be the first things, as well as a good water supply as well. This again depending on the skill of the group could take up to 3 years or as little as one.