Urine is practically sterile when fresh. If left out, it begins to grow bacteria. As long as you drink it immediately from a healthy person, I don't think there's much to worry about. I don't know how much is too much, though. The thing to keep in mind is that urine is the body's liquid waste, a byproduct of digestion and metabolism. I would imagine that if you didn't drink enough clear fluids you'd run the risk of building up uric acid and whatever else in your system. However, I'm not a doctor. It may be that your body wouldn't absorb anything but simply pass it on through.