(SPOILER)Have to have Celes, Edgar, Setzer. Two bosses (Tentacles and Dullahan).
Sabin doesn't require a boss (requires a scenario in Tzen). Cyan doesn't require a boss (requires going through Mount Zozo). Gau doesn't require a boss (get him on the Veldt). Mog doesn't require a boss (find him in Narshe). Gogo doesn't require a boss (get engulfed by that there Zone Eater).

So that's 8 characters making two full parties while fighting two bosses (which are necessary; just you try to get Locke without an airship).

Did I answer it right?