I'd say that you only need 3 powerful characters, that's what I did. Now there might be an occasion where this could be a small problem, for example if your three characters fall in battle you might not be able to resurrect even one of them with very weak character, this was almost an issue for me in the final boss battle, and I imagine that it could be little troublesome if your characters are much lower than what mine was (level 82), however I'm not sure whether the final boss is scaled according to player level, but I doubt it. My other characters were rather weak though, none of them were over lv 15...
But as a rule of thumb I would say that the three main characters are all you need to develop. The final boss has one REALLY powerful attack which often either kills 2/3 of your characters or the very least drops them to near zero, after that it's really crucial that you manage to heal/revie your characters fast enough, during the constant bombardment that comes from the boss, live or death can really be decided in a fraction of a second in those particular moments mainly depending on how long it will take until your next turn becomes available, in essence whether the boss or you get's the next move. You either have to manage to heal atleast one of your characters to max or bring out a benchwarmer that has enough health/gear to sustain one hit.
Personally I think you don't need to bring the benchwarmers to the table, but you might need a bit of luck in the battle if you don't, or at the very least you can't have any bad luck. I only died once in the final battle, but it was a close call on my second try also.
edit: If you die in the final moments of the boss battle, you lose some time as there are no save crystals for quite a long time before the battle.
edit2 Oh I wanted to point out that whether you need more than three characters is naturally related to the strength levels of your main guys, so if the top team is really strong they need less help and the reverse applies too, so basically I can only tell accurate information based on what I saw, so if your three characters are at a very high level they don't need help, but I'm not sure if they are at lv 50-60 I honestly don't know whether that's enough to beat the final boss.