The number of lights per foot of a Christmas tree can vary depending on the size of the tree, but a good rule of thumb is to use one to three strings of lights per foot of tree height. A small tree may only require one or two strings, while a larger tree may require three or more. It's also important to consider the wattage of the lights, with a higher wattage providing more light per string. When in doubt, it's best to start with less lights and add more as needed.