I'm wondering what would be the minimum number of people living on earth required to keep our current level of technology, e.g., in each field the state-of-the-art. Many technologies have long dependency chains, e.g., to run a nuclear power plant, besides the people running and maintaining it, it needs to be built at first. In order to build the power plant, we need to be able to produce steel etc. In order to produce steel, we need machines that can extract raw resources (e.g., ores) from the planet and also we need power in the first place to melt the ores, etc.
Besides these technically required resources, there are also other things required in order to maintain our technology level, e.g., we need people who can control the machines short-term, but also long-term people that are able to teach the required knowledge to other people, which in turn are able to build or run these things.
So, given all these partially multi-directional dependencies, what would be an estimate of how many people do we need to keep our standards?