The number of Christmas lights per foot of tree varies depending on the size and style of the tree, but generally, you should use about 10 to 12 lights per foot of tree for a medium-sized tree. For a larger tree, you may need to use more lights, while a smaller tree may require fewer. However, it's always a good idea to overestimate the number of lights you need, as you can always remove some if they're too bright. Ultimately, it's up to you to determine the perfect amount of lights for your tree.