It will certainly not only be difficult, it will be impossible in many restaurants if you expect to sit together. Some restaurants don't have seating for more than 8-10 at the same table.
We ran into this at Akerhaus last week, but they were able to accommodate us anyway. We had 10 people (one of them quite small -- 2 yrs), but they only have seating for up to 9. We tried to change the ADR, but no luck. When we showed up at the restaurant, however, I said 10 and they said, "Okay."
However, I don't think I'd risk being able to expand 8-10 to 13. I'd split the group up.
It's also pretty unrealistic to expect the CM's to magically put your group at adjoining tables. Sure, they will if they can...but the chances of two adjacent tables opening up together are pretty slim.