How much attention do you plan to give a project in any given week? (If the answer is "none," it's a Someday/Maybe and doesn't belong in your active list.) How much productive time do you have in any given week?
Productive time / average time per project = maximum manageable number of projects.
Alternatively:
Productive time / number of projects = average time per project
If you take on too many projects, the time you can devote to each will be too small. Since some projects will inevitably take more than the average, that means some will fall through the cracks and not get done.
Personally, I have a hard time achieving more than 8-10 productive hours per day. (Work and personal combined.) The rest is taken up with sleep, hygiene, food, inbox processing, and other stuff that doesn't directly move projects forward. That's 70 hours a week, so a maximum of 30-50 projects sounds about right.
Katherine