Spawn workers to use up nearly all available CPU

Say I have a CLI app that I want to do some work (e.g. process some media files). When I run one instance of the app it takes say 20% CPU. The rest 80% is idle. I want some sort of process manager that will run as many app instances as it takes to use nearly all the CPU up. The app knows what to do and how not to interfere with its siblings, but it can't spawn/manage itself. Is there any manager like that? I've been using supervisor to run fixed sets of different apps one instance each, but I don't know if it can be used to run variable amount of instances of the same app depending on CPU availability.