Evergreen trees, also known as conifers, have many different varieties. Conifers are by definition a plant that bears cones, so be sure to clarify if you have an evergreen or a deciduous variety. The shapes and sizes, as well as root structure, will determine how far apart they should be spaced when planting.
Pyramidal, columnar, shade and weeping are different classifications of evergreen trees. All of the varieties except the columnar trees require at least 15 feet of space between them. The Shade trees such as common hackberry, honey-locust, redmond linden, maple and bur oak can require as much as 40 feet in spacing. Columnar trees such as arborvitae and the tower poplar tend to grow more up instead of out, and should only need six to ten feet in space.
Creeping, globes and spreaders are the different types of evergreen shrubs. Spreading yews and birdsnest spruce are two types of shrub that only require 4 foot spacing. The other varieties require about 8 feet of space between each shrub, and at least 4 feet from a wall. These shrubs can also obtain the same height as width.
Using these numbers as a guideline should help you figure how many trees will fit in a certain area. Don't overcrowd your yard, and check with your local garden or arborist to obtain information about your area.