On one cruise, we were at a table for 18! I'm not sure why, but that's what they did (we hadn't specifically requested it.) I'd just make a request that you all be at one table. What they do is to place as many tables together as you need....I think the only thing is that each server handles about 24. For Huge groups (like 50), they do have to split the groups, but keep them in the same area (like 2 adjoining server stations).
Call your TA, and request that all 13 be at one table.