Crop load management on young apple trees is important to balance fruit load with vegetative growth so that "runting out" or biennial bearing does not happen. This is important with varieties such as Honeycrisp (in particular) that are prone to "runt out" and/or go into biennial bearing if over-cropped in the early orchard establishment years. Still, some crop is desirable beginning as early as the 2nd-leaf. Below are some suggested numbers of fruit based on measuring the tree trunk diameter just below where lateral branching and fruiting begins. (Usually about 24 inches from the ground.) This is based on the Valent/Cornell University Young Apple Thinning Gauge. There is also a link to an interactive crop load calculator that can be used to enter your own information. The Table and calculator are most useful during the 2nd through say 5th, 6th, and 7th leaf (roughly). Once trees have filled their space and begin full cropping, it's best to use the total desired number of fruit per tree based on tree spacing and desired yield per acre to define the desired number of fruits per tree. This is particularly true for tall-spindle apples.
* and other biennial varieties
** annual varieties including Gala, Empire, McIntosh, Delicious, etc.
Interactive crop load calculator on fruitadvisor.info