Corrected entry: General Grievous, whose two arms can split into four, has a total of twelve fingers (six on each hand when his arms aren't split). However in some shots (such as when he orders his ship's bridge crew to fire the emergency booster engines in an early scene) we can see he has five on each hand when his arms aren't split. In other shots, he has the correct number of fingers.
Matty W
20th Oct 2006
Correction: In the example that you mentioned, while General Grievous' right hand is mostly out of view, his left hand is seen, with six fingers on it. One of them was probably mistaken for another part of his hand, or a ship in the background.