If the question was....how many people are capable of building or constructing an AR-15-like weapon....I’d take a reasonable guess that more 25,000 machinists exist within the US who could probably do the job. Course, why limit our worries to people ‘with motivation’ with guns? How many PhD guys can construct a nuke bomb from scratch within the US? 10,000....100,000?
Nope. Anyone can do it. A Harbor Freight drill press is all that’s necessary.
My son’s built theirs. We used a milling machine simply because a friend has one. Great learning experience.
Others I know used simple drill presses.
Polymer stock is even easier than metal.
If you can smuggle people and drugs over the southern border, why not AK’s?