I have heard it recommended (from a good source, an ex elder) that you need to have at least 100 lights per foot of Christmas tree. So a 6 foot tree = 600 lights.
Different numbers of lights will have different effects. If you just want some sparkling of lights, for a 7-foot tree it only takes 25-50 well-placed lights. Large bulbs in small number will give you accent lighting.
A fairyland appearance can be had with about 500-600 mini lights, preferably flashing in random sequence. This effect is best with most of the lights on the surface, and lots of reflective and refractive ornaments. Fewer lights will give more of an isolated twinkling effect, where more densely lit trees will give more of that fairyland effect.
If you really want a showpiece similar to what you find featured in the malls, then I'm afraid you are going to need 2400-4000 lights for the tree. That is how they get those department store trees to look so good amidst the other trees and the bright lighting. If they twinkle, you can get a major fairyland effect. Just beware that for some, this amount of lighting can make a tree too bright. The tree will also appear to glow from the inside out if you put lots of lights inside the interior branches as well as the perimeter.
This is not a problem. Different people have different preferences. You do not want a brightly lit tree if you have Brother Hounder prowling around, or if it might disturb a sleeping baby. And you might prefer a sparsely lit tree. Just as there is no hard and fast rule of colors of lights (some prefer to put all clear or all red-and-clear; others like all colors), there is no rule about the amount of lights. Some people put clear lights on the interior of the tree and colored on the exterior to give more brightness. The most important thing is to know what to expect so you will end up with the effect you want without attracting unwanted attention from Brother Hounder.