You call them ASAP and try to change all the ADRs to 11 people. IF they can't do that, you keep the ADR for 10 and just show up with 11.
Chances are that they can just put another chair at the table. However, there are issues regarding maximum occupancy of the room and access to the exits which are controlled by the fire marshall--so they can't always allow another person, even if that person is an infant in a high chair. At peak dining times, they do get into total occupancy situations.