This can cause problems with balance, resulting in injury when trying to fly or land. For aesthetic reasons, some people leave the last one or two outside primaries and clip the ones closer to the body. This is not recommended for two reasons. First, a bird may be able to get a lot of lift from these primaries, especially if he gets loose outside and finds a gust of wind. Second, these feathers will have little support from surrounding feathers and are more likely to break.
Experienced people have a good idea of how many feathers to remove.
If in doubt, clip a few feathers. If your bird still retains too much flight capability, then clip a few more. Repeat this process until your bird can fly only a few feet. Some species of birds require clipping of more feathers than others. In general the more aerodynamic the species, the more feathers need to be trimmed.