The thinning of apple fruitlets early in the growing season is essential for flavor, size and color and much more.  In general an apple blossom opens up with 5 in each cluster.  Some varieties will have more flowers up to 7.  A fruit tree does not require nearly as many fruits as it will grow.  A natural thinning process takes plan in June, termed 'June Drop'.  This alone will never be enough to grow good tasting fruit, only to ease the stress on the tree.
 
Every variety should be handled similarly.  Apples need to be thinned out to the largest or "king" apple in each cluster and then spacing those out to 6inches appart.  This is approximatly the distance from thumb to pinkie when spread as far as possible.
 
Note:  While pulling off the unwanted apples be sure not to pull off the entire spur.  This is the little cluster of apples and the leaves at the base.  By snapping them off between the index finger and thumb most will role right off. 
 
Timing:  To maximize size of fruit the earlier they are thinned the better.  June and July for the mid-west are ideal times to remove the unwanted fruits.