100 Lights Per Foot is the Standard
Most people purchase 100 lights for each foot of height; this is the standard amount of lights for a Christmas tree. For example, if you have a 6 foot tall tree, you need about 600 lights for your tree.
Use More Lights for More Glow
If you want a more "sparkly" or brighter Christmas tree, buy at least 150 lights per foot or as many as 300. Another factor to consider: if your tree is wider than average, plan on using more lights per foot of vertical height; if your tree is thinner, you'll need fewer lights per foot.
Determine how many lights you'll need per foot of height by how bright and twinkling you want your tree: for a subdued glow, use 100 per foot; for a more stunning effect, use as many as 300 per foot.