Main Final Fantasy Series (13)
Final Fantasy I
Final Fantasy II
Final Fantasy III
Final Fantasy IV
Final Fantasy V
Final Fantasy VI
Final Fantasy VII
Final Fantasy VIII
Final Fantasy IX
Final Fantasy X
Final Fantasy XI
Final Fantasy XII
Final Fantasy XIII

"Final Fantasy" Sequels and Spinoffs (17)
Final Fantasy IV: The After Years
Crisis Core: Final Fantasy VII
Final Fantasy X-2
Final Fantasy XII: Revenant Wings
Final Fantasy XIII-2
Final Fantasy Mystic Quest
Final Fantasy Tactics
Final Fantasy Tactics Advance
Final Fantasy Tactics A2
Final Fantasy Legend
Final Fantasy Legend 2
Final Fantasy Legend 3
Final Fantasy: The 4 Heroes of Light
Dissidia Final Fantasy
Dissidia 012 Final Fantasy
Theatrhythm Final Fantasy
Theatrhythm Final Fantasy: Curtain Call

Kingdom Hearts Series (7)
Kingdom Hearts
Kingdom Hearts II
Kingdom Hearts: Chain of Memories
Kingdom Hearts: 358/2 Days
Kingdom Hearts: Birth by Sleep
Kingdom Hearts: Re:Coded
Kingdom Hearts 3D: Dream, Drop, Distance

Dragon Quest Series (7)
Dragon Quest
Dragon Quest II
Dragon Quest III
Dragon Quest IV
Dragon Quest V
Dragon Quest VII
Dragon Quest VIII

Other Square Games (9)
Secret of Mana
Chrono Trigger
Secret of Evermore
Super Mario RPG
Xenogears
Front Mission 3
Threads of Fate
Chrono Cross
Vagrant Story

Other Enix Games (4)
Paladin's Quest
Illusion of Gaia
Robotrek
Star Ocean

Other Square-Enix Games (5)
Sword of Mana
The World Ends with You
Tactics Ogre: Let Us Cling Together
Bravely Default
Bravely Second

18 total using strict criteria. However, it's pretty loose to call the FF Legend series a FF series so I'm going to give myself those 3 to bring it to 21. 4 Heroes of Light is also questionable since it is basically Bravely Default 0.

I'm pretty certain I'll be the only one to list Paladin's Quest and Robotrek.