In most cases - and that’s the professional scenario - you should make a Harmony scene for each planned shot, that is, each camera setup, as you would do in live action (you won’t ‘edit’ on the camera). That way you’ll have your project better organized and it will be easy in editing to make changes such as minor cuts or switch the order of shots, something that Harmony isn’t exactly meant for. Also, if you make many shots ‘edited’ on a single Harmony scene you might have issues in your system due to the size and/or complexity. Finally, if for some reason you have a file corrupted you won’t have the latest version of a shot compromised but the latest version of the whole project (backup often in any case).
In some cases, for instance, some sort of complex mix of two shots that would be easier or more convenient to do at the animation/compositing stage I would consider doing it in Harmony.
If you have a storyboard, even if very rough, it won’t be complicated to track the project, especially since it’s just 20 shots. You can use Excell or Google Sheets and put an image of each shot with the stages of production in different columns if you want to track and organize well your work.