Some friends of mine who practice meditation (Zen and Sufism) focus on one thing, visualizing one thing, and in turn are supposedly, with much practice, able to perceive many things. My very limited understanding is that focusing on a single thing quiets the mind and enables it to--eventually--perceive the nature of reality: everything.

Personally I don't think there is a specific number that applies to all humans. I think it would be different depending on the perceptive abilities of each individual. And the phenomenon I described above suggests that it is quite a bit more complex in yet a different way.

I am curious--why are you pondering this?