as far as I know, you have to call Disney Dining to get a table for 9 - the system does not accept any number higher than 8 for that reason. Your other option is to split your group if you don't want to call. And you may have to do that anyway since there are few tables set aside for that large of a group...
I really would not try to just show up with 9...depending on the table set-up there, it could very well be that there is no way to pull up an extra chair (thinking about some of the places we've seen round tables where they crammed 8 people...they didn't really fit and there's no way they could have squeezed another chair around that table.) You do take the chance they'll flat-out tell you no, they can't seat 9. Or tell you there will be a 1-2 hour wait.