Except your forgetting that Gabranth is shown in the trailer. So having one slot has to go to one of them but both are confirmed...

I'm sure SE is just teasing. 10 new characters sounds about right and if memory serves me correct, they gave that estimate after Lightning, Kain, and Tifa were already confirmed, so there may even be a chance the list of new characters are actually 13.

If it's 13 then that is enough to bring one new character from each FF title, unless SE decides to screw a few entries (I, III, and IX sadly come to mind) so they can bring out more characters from more popular FFs or spin-offs.