How often was this Aeon summoned?

They never say Lord Zaon was an aeon available to other summoners. He was just a Final Aeon like all the others. I will attempt to clarify the aeon process a bit so you understand.

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Creating an aeon requires a living person to become a fayth. The bond between summoner and fayth creates the aeon. The strength of the bond determines the strength of the aeon that is summoned.

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A Final Aeon follows the same process but the fayth is created from a person with a very close bond to the summoner. That bond is what makes a Final Aeon so powerful. They are called Final Aeons by the people of Spira, most of whom don't know what is really going on. From their perspective these magical Final Aeons can only be summoned one time, then they kill the summoner. Pretty final.

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In reality, the Final Aeon is final because after destroying Sin, Yu Yevon comes out. He uses his super summoning powers to hijack the aeon that just destroyed Sin. He uses it to kill the summoner then spends some period of time corrupting the aeon, transforming it into a new Sin that Yu Yevon can use as a host.

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Getting back to Lord Zaon and Lady Yunalesca, she created turned Lord Zaon into the fayth you see in Zanarkand. He would have been hijacked by Yu Yevon to create Sin, and when another summoner came along and killed that Sin, that also killed the fayth.

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You have hit on a bit of a plot hole, which is where are the other fayth? A thousand years of summoners coming to Zanarkand and using their guardians to create Final Aeons means Lady Yunalesca probably has a closet full of those things.