Well I see where you are comming from. You do see a lot of people you would never remeber. All it takes is mixing up 2 people you seen and you got a new person. So if you seen a 1000 people you could easily have unlimited amount of people you can make.

A few small changes to a person face and they would look way different. Now if your brain takes someones face and change it, or if they just make up a new one, I don't know. I think the end result is you can have a person looking any way you want.