Is this a group booking or several individual bookings? The dining room management will try to put the group at tables adjacent to one another and will do the same if these are individual bookings that are cross-referenced. Are you trying to do this yourself or are you using an agent? I recommend the latter given the group size.
Carnival's automatic gratuities are now $11.50 per person/per day, and of course the 15% added to drink purchases.