I've only got one pair of hands
1. There is a finite amount of things I can pick up, carry, or do with my hands. A: "Don't forget to carry the groceries in from the car." B: "Can you come out and help? I'm bringing the kids inside, and I've only got one pair of hands."
2. I am too busy to take on any other work. The boss wants me to go over all the intern's work, on top of what I'm already trying to finish for Friday. I've only got one pair of hands!