Ared blood cell is in an artery in the right leg. how many capillary beds will it pass through before it returns to the left atrium of the heart?

It will pass through one capillary bed that is part of the systemic blood circulation, the lower body one. Then, the veins will direct it to the heart in the vena cava, where the red blood cell will be pumped with the surrounding blood to the lungs so it becomes oxygenated again. There are more capillary beds in the systemic circulation, but the blood flow is mantained by some valves that won't let this red blood cell break it. That's why is only goes through one way.