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just to shed some light on the the number of processes issue: calibre maintains two separate worker process pools. One is used for adding books/saving to disk and the other for conversions. So if you set it to 6 that means a maximum of 3 conversions will run simultaneously. And that is why you will see the number of worker processes changes by two when you use the up and down arrows.
In addition to this some conversion plugins run tasks in their own poll of processes, so for example if you bulk convert comics, each comic conversion will use three separate processes to render the images. The job manager knows this so it will run only a single comic conversion simultaneously.
And since I'm sure someone will ask: The reason adding/savig books are in separate processes is because of PDF. PDF processing libraries can crash on reading PDFs and I dont want the crash to take down all of calibre. Also when adding EPUB books, in order to extract the cover you have to sometimes render the HTML of the first page, which means that it either has to run the GUI thread of the main proces or in a separate process.
Finally, the reason calibre keep workers alive and idle instead of launching on demand is to workaround the slow startup time of python processes.
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