As for hobbies, I think I would only do one essay about all your hobbies, not one essay for each hobby. I did not include any hobby essay in my app, but I did mention them within each essay. For example, one of my jobs has an extra perk of taking photos, so I mentioned how I love photography and used this time to further my skills. I just added that subtle hint about photography and it actually came up in an interview. You want to talk about any jobs, research, volunteering, clubs, leadership positions in this section. If these involve or develop your hobbies, then mention them there. Also, I believe you can fill out up to 15 or so, but don't feel the need to fill out 15. If you have 7 or 10 or whatever, that is fine.