I think the number of subjects or characters you can present in a give amount of time is going to vary tremendously from project to project, and is going to depend a lot on your own skills and objectives as a filmmaker. One of my best films was a scant 9 minutes long, and featured seven subjects, selected from perhaps 30 interviews. My latest film is 54 minutes long and has only two subjects, interviewed simultaniously.
Again I can't help but think of the idea of tangency and overlap, only this time between what you think the finished film is supposed to be and the film the universe want you to make. You can't cede control to fate, but neither can you can ignore the life a project will take on of its own accord – nor would you want to!
Ultimately, a film's "will to be" is you best ally. As much as you should strive to be in a dialog with your subjects, you should hope to find yourself in a dialog with your film. When/if that happens, the film will tell you the right way to arrange your elements at least as much as you will decide.
When do we get to see it? :-)