Well I didn't start with 100, for sure. I still don't have 100 items. :) I think I had like 10 or so when I started, but I kept adding listings over time. I don't think it matters how much you start with, just keep up on updating your shop daily. But if you're like me and have other responsibilities (work, classes, boy, etc.), then just as much as possible. Post a couple new postings per day to keep your shop in the searches and in the "recent posts" sections. Also, keep promoting your shop in the teams and treasuries.
Like I tell my friends who want to start an Etsy shop: just start. Don't worry about the details right now. Those can be ironed out later. Just get your stuff out there as soon as possible so that customers can start looking at your products. That's my suggestion. Good luck with your shop!