Final Fantasy VI > Chapter 12: Kefka's Tower
You will be able to use at most 12 characters in the final battle with Kefka, and against all the dungeons and bosses preceding that point. All your groups will engage in their own boss fights and quests so you will need at three strong parties.
It should be noted that if one of your parties possess the Moogle Charm and equips it on Mog that party can explore Kekfa's Tower without worrying about anything but boss fights. This may be able to be used to protect a weak party early on in picking up equipment, but in general you will still need three strong parties to complete some areas where the path divides.
It should also be noted that due to 2 extra characters being placed in the game, Umaro and Gogo, you will have two characters which will not be able to be used in the final battle. In short, you should decide well in advance to fighting in Kefka's Tower which two characters you should choose to ignore and not level up at all. The following is some of my consideration for doing so:
(when I say a character is "required" for a particular task it means that it's unavoidable for using them in a particular scenario which normally involves boss fights and levelling up)
Celes-Required for all the early quests up until you get the airship, has strong magical ability.
Edgar-Required for early quests in the world of ruin as well, his strong complement of tools and the chainsaw make him invaluable.
Sabin-Also required for early quests, his blitzes and the bum rush make him invaluable. Setzer-Required for the quest in Darril's tomb. Possesses one of the weakest special abilities later on in the game, although GP toss can be valuable.
Locke-Required for looting Narshe, and a good deal of the missions in the world of balance.
Cyan-Strong physical attacks, and the swordtechs are very useful when you have the time to charge up.
Terra-Very strong magical ability, she's practically the main character anyway, and she can morph into an Esper.
Mog-Required in order to complete the sidequest to recruit Umaro. His dance skills are useful but unpredictable sometimes.
Strago-Required to complete the Hidon sidequest, strong magical ability and useful lores.
Gogo-The most flexible character in the game, never needs to be trained in magic, and can copy any characters moves and techniques. Absolutely essential.
Umaro-Strong physical attacks and a force to be reckoned with when he has the rage ring or blizzard orb.
Shadow-Not required for any quests, but used for the floating island in the world of balance so he may already be levelled up enough to justify keeping him.
Relm-Not required for any quests, strong magical ability, Sketch is a little dangerous to use because of the bug.
Gau-Not required for any quests, rages can be useful early on but they can be a hassle to figure out.
In short, I recommend ditching Gau and Relm because you likely won't be using them anyway on any of your required quests and their abilities aren't as useful as some of the others. You could also make a case for leaving out Setzer or Shadow because of their special abilities, and that after a certain point the built in "abilities" of the character don't matter as much since you may chiefly rely on magical attacks.
Anyway, you will need to split your party up into three teams.
Your strongest physical attackers are: Edgar, Sabin, and Cyan. Edgar and Sabin both have physical attacks which can easily do 9999 damage and Cyan has powerful multi-enemy physical attacks when given time to charge up.
Your strongest magic users are: Terra, Celes, and Strago. Terra, Celes, and Strago have the highest magical ability out of the characters I chose to keep so it makes sense to split them up and teach them the most magic.
"Wildcard" characters are: Shadow, Locke, and Gogo. Shadow and Locke should normally receive the Genji Glove and one of them could get the atma weapon or offering to make them even more powerful to compensate for their lack of innate special abilities. Whatever team gets Gogo has access to the skills of any other character and all the magic known by other characters in that party, so it will be strong enough to compensate for other disadvantages.
Secondary attack characters are: Mog, Setzer, Umaro. They all have decent special abilities but are generally not as reliable. Whatever team gets Umaro will probably be strong enough to compensate for other disadvantages if you give him the rage ring or blizzard orb.
From the previous list you should give each party a strong physical attacker, magic user, wildcard character, and secondary attacker. Gogo and Umaro can be kept on separate teams and you can do your own job of balancing things out. In general each party should also have at least one person who knows Ultima or an extremely powerful magic spell, and each party should be able to cast strong healing and life spells.