You're probably thinking that Mickey Mouse only has FOUR fingers, and you'd be correct. Disney explained that the reason Mickey only has three fingers and a thumb is for both artistic and financial reasons. Disney said if Mickey had five fingers, his hand would look weird, "five digits are too many for a mouse, it would look like a bunch of bananas."
The second reason for Mickey only having four fingers is financial. Disney also said if you multiplied the time that animators saved by not having to draw a fifth finger in every one of the 45,000 drawings that make up a 6 1/2 minute short film, you would reach the conclusion that the studio saved millions of dollars. All of that just for a little finger!