Make reservations for your party of 12 in the dining room of your choice, and the time of your choice. The app will indicate something like "various tables", instead of "private table for 12". The first night tell the Head Waiter your wishes for same tables and servers. This usually will take care of your needs. I would do this soon after the dining room opens.
About a year ago, we had a Head Waiter was not too cooperative for our party of eight, although we had the same table five of the seven nights. The cruise before that we had a party of 14 and that Head Waiter went out his way to get us a table for six next to a table for eight for the whole week. We also set some guidelines for seating. You could not sit next to the same person as the night before, except for your significant other. Also, change tables once in a while.