Dry yeast yields approximately 20 billion cells per dry gram, when properly rehydrated, or half that when sprinkled dry directly into the wort.
Pitching the correct amount of yeast is ideal, but between over and underpitching, in my opinion, underpitching is far worse. Overpitching seems to have far fewer consequences, particularly when the fermentation temperature is tightly controlled. I always tend to comfortably overpitch, rather than risk walking too close to the line and accidentally underpitching.
If you're using US-5 and properly rehydrate it according to the manufacturer's directions, you can count on that packet of yeast delivering approximately 200 - 220 billion cells of hungry yeast.