Cheryl:

Columnar apple trees are a novelty item, developed to sell nursery stock to people who lack the space for in-ground trees and want to try container growing. They are not for producing apples.

I have seen a very few cases where people report growing as many as 10 or a dozen apples on these trees, but most people who try them are disappointed. My personal view is that, if you truly don't have space to plant in the ground, and have a very large container, planting a branched tree on a full-dwarfing rootstock like M-27 is a better alternative than the columnar trees.

Don Yellman, Great Falls, VA