The amount of fruit which starts to grow depends on how lucky you were with good weather when the tree was in flower, and how many pollinating insects found it.
If you have literally "dozens" of small apples, most of them will drop from the tree naturally in the next few weeks, as the tree gets rid of the excess fruit that it can't grow to maturity.
Instead of just letting nature take its course (which can depend on random factors like temperature, rainfall, etc), you will get a more predictable result and better quality crop by reducing each cluster of fruit to just one or two apples.
There are two problems will the tree trying to produce too many apples. First there is a danger that the weight of the crop will break the tree, and second the tree may get into a rhythm where it tries to produce a big crop every other year, instead of a sustainable size crop every year. Once that two-year rhythm has started, it can be difficult to "reset" the tree to cropping every year.