Here are some “before” and “after” shots of my Red Baron peach tree that I thinned yesterday. My rule of thumb: remove all but the largest specimens, and only keep one peach for every 6 inches of branch length. NEVER keep two peaches on a branch that are coming off at the same node. If you do this type of thinning, the fruits will be much larger, more juicy and of course, better tasting.
Young trees should be thinned even more aggressively the first and second years, to force the energy into establishment of a strong root system. I keep only one peach per branch, two if the branch is especially thick.
Another thing to notice is whether the branch is hanging low to the ground due to the weight of the fruit. If so, please consider whether the branch may break if you don’t remove more peaches.
If you have a tree that is especially fruitful, it may be because it has a special rootstock called Citation. Other types of rootstock may not provide as much fruit and won’t need as much thinning, but all my trees are on Citation for its wonderful characteristics: precocious production and ability to handle clay soil.