7 has a movie about it =D
i think <<;
it is about the game right? >>;
It's a sequel
It has two sequels and two prequels (three if you count 10 Normura stated that it was)
Tactics/12 has more I think ._.
5 has an Anime
And 4 has a cellphone game
And 10 has 10-2
None of them needed the sequels (10 sort of needed it)
All the others haven't gotten one yet because it would be hard to (At the end of 6 all the magic disapeared)
I don't know about the rest
Though you might be able to conect some things from one game to the other (6 and 12's summons are both called Espers and they both have Magicite, plus some themes are the same)
Only two that are officially been connected are 7 and 10