I have a similiar problem. I've made all my ADR's for 2 people and now another person has just been added to our vacation package. I was told when I called it shouldn't be a problem but they recommended that I call and change all my ADR's for three since the table I'm booked on could be a table for 2. The difference between 12 and 13 is so minimal I doubt you would have a problem , but I would call just in case.
I had to add to my ADR's also. I would rec calling and adding the extra person. WE had to change some of our days because they were not able to accomodate the extra person on our origional date.