Interesting that you should ask. . .I just spent two days trying to book seatings for my group.
Originally, my group was 12 (9 adults, 1 child, 2 infants). No problem to be seated at Cinderella's Royal Table, Hollywood & Vine/Fantasmic, Marakesh, or Le Cellier.
The next day, someone else decided to join the trip. I was told that my group size of thirteen meant that I would have to book seatings through group reservations. Well, the result is that we're split up at Royal Table, Hollywood & Vine, and Le Cellier (at Le Cellier, we're split 4, 4, 3, and 2!). Marakesh will seat us together, but Marakesh won't seat "groups" (that is, more than 12 people) after lunch, so our dinner plans had to be changed.
So, it will be inconvient for a big group. Here's how I decided to make the best of it: we're a family group that doesn't spend a lot of time together, so I went ahead and decided to split the tables and assign seats in advance--people alternate who they sit with, so that they get to visit with different members of the family. At Royal Table, this means that I won't get to sit with my son, but his grandparents will really enjoy the chance to share the meal with the grandkids. And, at Le Cellier, my sister and brother-in-law will get a chance to spend time with their nephew and my husband and I catch up with my parents.
Good luck with your trip.