I would say, op is right when learning by hand. Managing more than say 3 books at once or more than 1-3 subjects is impossible. Tackling too many things at once is easy recipe for madness. (these numbers are vague estimates and not from experience; I have near 0 non-supermemo decent learning experience)

When it comes to SuperMemo though, you're not managing learning. You more put in what you want to learn, push a lever or two (priority/intervals) and supermemo takes care of the rest. SuperMemo is way less stressful than normal learning because when you use it and are doing reps, you can focus as much as you want on what's in front of you and move on when you want to. The costs of switching between things are near 0 since you'll always see the thing again (unless you decide not to).

For working on more than 1 project at once, I think that's pushing it though with incremental writing (https://supermemo.guru/wiki/Incremental_writing) it becomes more feasible to work on multiple things in parallel incrementally (also enjoyably).