Long before computers and even the printing press, there were communities with "thousands and thousands" of laws. I don't know that a Third World War will suddenly resolve all of that to a handful.
I think a good example would be various occupied cities and nations in Europe during and immediately after WW2. As a general rule, the 'basic' laws prevailed, but the 'occupying power' would unilaterally suspend some, add a few of their own, etc. These might encompass
- forced labor "for the good of the community"
- expulsion of persons for any number of good or (more often) bad reasons
- control of production, resources, and markets; confiscation of goods
- control of movement: visas, travel permits, "papiss pleez" in general
- control of society: rules forbidding fraternization, for example
- prohibitions on weapons, or the formation of compulsory militias
- suppression of religions, political groups; prohibitions of languages, particular costumes/uniforms
- penalties on persons based on their national origin, military service, languages, social class
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Michael B.