Each of your Cooks will start off with 3 preparation areas but they can all be upgraded to 4, 5 or 6 areas each, but at a cost! To add a preparation slot, touch the helper so that the ‘add slot’ shadow appears (seen in the images below).
You will see that the first slot will cost you 5 Gems. The second will cost an additional 10 Gems and the 3rd slot will cost 20 Gems. (they used to be 10, 20, 100 when they were called Helpers, as seen in these images).
You may notice when you visit other players that they have more than one of each of these Cooks. Earlier versions of the game allowed you to have an extra of each Cook but was removed early on in development. Players with more than one Cook were early adopters of the game. Lucky them huh?