I think that its awesome you are getting an early start in thinking through this. I started working on mine in May. Don't try to actually start entering your information online in AMCAS yet though, because they have not yet switched the app to the next cycle (and most of your entered info will be dumped when they switch). Yep, I made that mistake and had to start over.
15 slots, complete with a respectable amount of space for a description of each one. You do not need to fill up the space, type as much as is appropriate for each activity. Some will only require one sentence perhaps, others may take a long paragraph depending on your style and what you have to say. Good luck!
Hey. I stressed out alot on how to input my activities. When it comes down to it though you have to realize that it will be read by poeple who are probably alot smarter than you. So as long as its clear to you and maybe some friends it should be fine. I left "average hours per week" blank on most of them and then explained it in the description.
I spent more time on my descriptions then on my personal statment.