I often have more than 10 kids start the season with us, as several of them tend to fizzle out before competition - especially if they have never done it before, and they think they will be playing with Legos! It usually works out. My Plan B is to split the team in half, and have two smaller teams compete. I've had them work side-by-side together, which works out just as well.
As far as the roster goes, you will register your (up to) 10 team members in October, right before competition. In my experience, you will have experienced your drop-outs by then. If the child doesn't mind not competing, then don't add her to the roster. I tend not to turn away kids who are really interested and committed, because learning is learning!