Apple trees are not self-pollinating. This means that if you want your apple tree to bear fruit you are going to need plant two apple trees so they can pollinate each other. If you only have one apple tree in your yard then your apple tree will only produce beautiful blossoms but no fruit.
There are exceptions, if your neighbor has an Apple Tree and you have an Apple Tree then those two Apple Trees are growing close enough together that they will pollinate each other and it won’t be necessary for you to have two trees in your yard.
Typically if there are two trees within 100m of each other than they will pollinate each other.
Combination Apple Trees are Self-pollinating
You can also purchase a Combination Apple Tree. A combination apple tree is an apple tree that has branches from other varieties of apple tree grafted onto it. This means each grafted branch of the tree will produce a different variety of apple. Since the grafted branches are genetically different from each other this tree will pollinate itself.