In a heavy crop year, we clean out all the peaches in the crotch angle first. Those peaches grow into the shoots and get damaged easily when picked.
Next, we thin are any doubles (Siamese twins), catfaced or otherwise damaged fruit.
Then we thin off one of any pairs of twins. Twins are where there are two peaches at one node. Sometimes there will be three at one node. More than one peaches at a node results in smaller peaches, so we thin those nodes down to one peach per node.
We also try to keep some distance between the fruit. Just like multiple peaches at one node produce smaller fruit, peaches too close together also produce smaller fruit.
We try to leave the biggest fruit, and thin the smaller fruit, since the bigger fruit typically have a larger cell count.
We also try to avoid peaches right at the tip, if possible. Peaches are mostly fed from the top down, so peaches at the very tip typically aren’t as large.
For an 18" shoot, we would ideally leave one maybe 4" from the tip, and 4" from the base. In good years we don’t grow any fruit on shoots less than 8" long.
I also prefer to remove the peaches from the top of the shoot and leave the ones on the bottom. Seems like birds will damage the fruit on top of the shoot slightly more than fruit on the bottom, since fruit on the top has easier access for birds. But this rule is pretty low on the priority list.
In lean years, we grow peaches on short shoots. We also keep doubles and just break off one of the Siamese twins. One of the twins can be carefully broken off early on. It will heal and produce a good peach, but will look slightly different than other peaches. We also keep peaches in crotch angles in lean years.