That is really a hard call, especially if you are going to wrap real trees, bushes, or hang them from the roof outside. You will just have to guess at the length you will need and look at how long the light strings are to determine the proper number to buy. For a Christmas tree, you could try to use regular string or light rope to wind around the tree first before ordering your lights. This will give you a rough estimate of how many packs to order. You can really never have too many lights because there is always somewhere to put them up.