>>> So, how many FF Spin off games are out there exactly?
Any hidden gem among the more unknown ones?

Here is a list I managed to get, I guess there are even more.

Crystal Chronicles
Chocobo games
Compilation of Final Fantasy VII
Crystal Defenders
Final Fantasy Dimensions
Final Fantasy Mystic Quest
Final Fantasy Tactics
Final Fantasy: The 4 Heroes of Light
Final Fantasy XII: Revenant Wings
Final Fantasy IV: The After Years
Final Fantasy Type-0
Final Fantasy Adventure
Theatrhythm Final Fantasy
Final Fantasy All the Bravest
Final Fantasy Artniks
Pictlogica Final Fantasy
Itadaki Street