Often, but not always. Most fruit trees need to be pollinated with at least two or three compatible trees. This means if possible, you should plant a couple of trees to encourage pollination, giving them about 50 feet of space between the trees.  However, if you are in an urban area and there are neighboring yards with apple or crabapple trees, you should be OK to plant only one. (My neighborhood is filled with apple trees, so it isn’t an issue for me to have a single tree in the backyard.)  The chance of neighboring trees of other fruiting varieties (peach, plum, cherry) may not be so high, so either check with neighbors or err on the side of caution and plant a couple.