I believe you're talking about the 65 episode rule Disney had in effect in the early-mid 2000's, where no show on the network was to go beyond that number of episodes. Of course, there's been exceptions with shows like Kim Possible and That's So Raven, just to name a few. In fact, those shows were the reason the rule was (for the most part) abolished, since they were really popular.
But in reality, the cut-off point of 65 episodes has more to do with programming schedules than any personal feelings about a series on the part of studio executives. With 65 episodes, one episode can be broadcast each weekday, reaching the 65th episode at the end of the 13th week (5 x 13 = 65). Thirteen weeks is one quarter of a year. Four 65-episode shows can be broadcast in a calendar year.
The 65 episode rule isn't just a Disney thing either, it's been done on other shows on other networks too, as it's the minimum number needed for a show to be syndicated.