The trees can be planted anywhere between 2 metres and 6 metres apart. Two metres if you are trellising them and using dwarf trees, apples, quinces etc.
If planting in a standard orchard situation, you can plant anywhere between four to six metres apart. If you plant close, the tree roots will meet each other sooner, and start competing with each other, thereby keeping their size more limited. If you plant further apart, you will get bigger more spreading trees, and in most situations they will grow to fill up the space in between.