Ah, you really should...anyways...*cough*
Spoiler:
There are no summons because...
Yuna never receieves the Final Aeon, for when she gets there, she learns that it has been dead for quite some time and that she must choose one of her gaurdians to become the final aeon. Of course, this gaurdian must give up there life and after killing sin, they would become part of him. So Yuna, refuses to choose this path and they end up fighting Yunalesca, the first summoner to defeat sin. After doing this, they must find another way to kill sin.
The aeons speak forth and they tell Yuna to call upon them
in battle with the controller of Sin, Yu Yevon.
While they are called, Yu Yeveon posses them, so Yuna and friends must kill the summon, and so on.
Someone please, I'm really not good at explaining things ^^"