If you are really going to get into the characters, I would say a lot. Because there are at least 50 characters that I can say off the top of my head at the MK alone. If you are going to only do the occassional character, you probably won't need a lot. Do you have any specific characters in mind that you want to meet? Maybe buy a notebook that comes with around 100 pages. Or go bigger or smaller, depending on what you want to do.