Well, theirs (SPOILERS ON WHICH SUMMONS THERE ARE IN GAME) :
(Im sorry if the spoiler thingy doesnt work, im new)




(SPOILER)
Odin
Pheonix
Bahamut
Neo Bahamut
Bahamut ZERO
Knights of the Round
Alexander
Hades
The Wind One ( I forgot its name)
Ramuh
Choco/Mog
Carbuncle, I think
Titan
Ifrit
Shiva
Leviathan
Kjata
Master Summon



Im pretty sure I missed some, but that equals to about 18 summons