The most vague and commonly asked question, the server specifications depend on your needs and on your sites.
It is possible to have a server with 1 site on it with the load averages sky high and the same one with 500 other sites with low load averages, there are a lot of variables into this, including the budget, so I would do my own research and see how everything works out. I also suggest that once you get a setup that works for you (CPU, HDDs, RAM, etc), stick with it for new servers until it's starting to get old.
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