From The TV IV

Disney's 65 Episode Policy is a commonly referenced company guideline for television shows produced by The Walt Disney Company. The policy is not official, but is perceived to be a standard practice that limits television shows to 65 episodes (a total of 13 weeks' worth of episodes if aired one episode per day Monday to Friday, with three additional airings over the course of one year), to prevent stagnation and to help increase the amount of shows in the Disney repertoire.

The 65 episode limit is not unique to Disney, and is in fact practiced by many animated programs. This is most likely due to the fact that 65 episodes is considered the minimum number needed for syndication.

Some examples