FYI there are 7 party members not 8.
Well it could very well be 8 or more...and they just didn't reveal the character yet. I mean traditionally dating back even the first Star Ocean the entire team you could recruit was comprised of 8 characters total, same for SO2, 3, and 4.
Also there is factoring alternative characters joining. Like in SO3 choosing between Nel, Albel, Peppita, and Roger. Just the same Star Ocean 2 you could choose between Celine, Ashton, Dias, Opera, Ernest, Precis and Bowman, and whoever else I'm forgetting.
I mean they could just be showing off currently 7 but later on you could possibly have a max of 8 like the previous games as well as being able to alternatively recruit different characters based on Private Action responses. I'd expect tri-Ace to keep some things a secret at least.