A new energy source is invented using the energy of the void and it takes the world by storm. By the time they realise it creates connections to other dimensions it is too late, and nightmarish+fantasy creatures will sporadically spawn in and wreak havoc.
A spawning can range from a few individuals to hundreds of thousands. These spawnings can happen anywhere, and while you have seconds to a minute warning by some sound and ripple effects in the air, it is rather problematic if one creature spawns in your bedroom while you try to sleep for example. The creatures are unpredictable, and can be passive and suddenly attack anything in sight. While moving things are preferred, they have no problem attacking lamp posts, doors, walls, roads, trees etc.
As another side-effect of a spawning is that for hours after a spawning, radio waves stop being able to produce understandable messages and any sub 0.2 micron computer component stops functioning properly, stranding the technology at around 2000 level of technology.
Due to the danger posed by these creatures both the infrastructure and humans have been decimated over time. To survive people are banding together and restructuring their society, losing access to much of their ability to transport people and goods or their agricultural food production.
To ask my original question I need to know the minimum requirements for a large group of people to survive, create weapons (preferably firearms) to fight off a spawning and maintain a city environment. Lets express it in minimum requirements per X population, say 100.000 people. You are allowed to express it differently if you want. For this question it does not matter how those minimum requirements are met, only what they are.
For clarity the question is: what are the minimum requirements for people to survive and maintain themselves in a city environment?
I'm looking for for the most essential things like clean water.