Another important aspect of how many fruit trees to grows is the requirements of the fruit trees themselves. Some require cross-pollination and therefore may need more then one of each variety in order to produce fruit. The place where you buy them will know so don't forget to ask. Another thing you will have to take into consideration is whether to get the draft variety or the standard variety of fruit trees. Sometime costs is a factor You can usually grow more dwarf trees in the same area as a standard tree.
Dwarf trees bear sooner and are also easier to prune and harvest fruit from.
When planting keep in mind how they will look when full grown and give them the room they need. You can plant them closer together but this will require more care on your part. Think about installing a drip system instead of using over head watering. Uses less water, controls diseases and pests. Natural sprayings should be done as needed.