How often does eye damage occur due to a corneal abrasion?

Rare. Most abrasions are small, superficial, heal quickly without sequelae. They are quite common, but eye damage usually doesn't occur. An infection can make things worse.
Rarely. Usually, injuries which are limited to the corneal surface (epithelium) heal quickly and completely. Sometimes, recurrent corneal erosion (a reopening of the of the original injury site) can occur so be sure to use lots of artificial tears.