Most fruit trees produce heavier crops when pollinated by another variety of the same type of fruit flowering at the same time.Choose apple trees for example with same or adjacent pollinate groups to achieve the best crops.
For example Sunset in group 3 will pollinate any other apple variety in group 3 or those in group 2 or 4.
Some varieties are good pollinators ,e.g.Greensleeves, James Grieve.
Alternatively use flowering crab apples to increase the amount of useful pollen in the orchard.
In large orchards plant one crab apple for every 20 apple trees. The crab apples can be closely spaced and pruned hard after flowering each year.