Post by using a number as the premise.
It's simple.

Sample

One Ultima Weapon in Final Fantasy 7.


Like that

Two extra characters in Final Fantasy VII
Yuffie and Vincent

Like that?

Three people who already know magic in Final Fantasy 6
Terra, Celes and Strago

Four Fiends of the elements
Milon, Kagnazzo, Valvaris, Rubicant

Four crystals of myth
Fire, Water, Wind and Earth

The five Crystal Warriors from FFV. (though there's only 4 at first )
Butz, Ferris, Reina, Galuf, and Krile.

I like using the Japanese translations for almost any game. I think many r a whole lot better than the English version names. That's why you see the name "Butz" and not "Bartz", though "Bartz" is actually a better translation than most, so I'll give it credit.

Six Main Characters in Final Fantasy 8
Squall, Quistis, Rinoa, Selphie, Irvine, Zell

Seven is the number of luck for Cait Sith
Especially when they come in triplets.

Eight Crystals in Final Fantasy 4

Nine Letters in Bladebeam

Ten main elements in Final Fantasy

Fire
Ice
Lightning
Water
Earth
Wind
Gravity
Holy
Dark
Poison

Eleven incarnations of Bahamut

Bahamut (Dragon King) - FF3
Bahamut (Summon God) - FF4
Bahamut (Superior Summon) - FF5
Bahamut (Ancient Esper) - FF6
Bahamut (Power Esper - FF7
Neo Bahamut
Bahamut Zero
Bahamut GF - FF8
Bahamut Garnet - FF9
Aeon Bahamut - FF10
Bahamut Dark - FF10-2

Twelve original Zodiac Knights
in Final Fantasy Tactics

^ Thirteen Zodiac stones in Final Fantasy Tactics.
The final 13th stone is held by Elidibs

14 Types of summon in Final Fantasy 4
{Pretty good }

Choco
Imp
Bomb
Mage
Cocatrice
Ramuh
Ifrit
Shiva
Titan
Sylph
Asura
Leviathan
Odin
Bahamut

Fifteen is a planned Final Fantasy game via Square buying the rights to the webadress.