There is no set of animators for a One Piece episode, Before the shift to Wide Screen, it was rare to see an episode with more than 10 Key animators, but I haven't checked all of them to be sure. Now I see from 3-20 animators per episode, and depending on the importance of the episode, the number of the animators vary, and this causes consistency issues. When there is many animators on one episode, each one gets to do less scenes, hence better performance. But in the case of less animators the animator's performance get affected, for example, episode 482 it had around 5 animators, and Nakatani did the key animation for most of the important scenes, but when it came to the final scene(Akainu Penetrating Ace) her performance was clearly affected, and the scene felt half done.
Now, It's just random, the animators keep shifting around from one AD to another, save for some, like Nakatani she usually works in Tate's episodes, and Hitoshi Kamata is one of the main animators that work with Ishizuka. There are more animators that usually work under a specific AD, but I was just using examples.