So I remember during my last playthrough on the HD remaster a couple of years back, wondering how long the Calm lasts. Is this ever clarified? I know there have been 4 high summoners prior to Yuna, but there's also been 1000 years of Sin. So that's one summoner every 250 years.
At first you might think that this is because the calm lasts for a couple of hundred years, but since Braska was the last one and Sin is already back, that throws that theory out of the window. So for just under 1000 years there were only 3 calms? If they last around 10-20 years then why weren't there any other High Summoners stopping Sin? Why aren't there a few dozen?

This brings me to my next question. I seem to remember them hinting in the game that the final summoning doesn't necessarily mean that Sin will be defeated. That it's just the only chance to defeat Sin in battle, but doesn't always work. Is this ever confirmed? It would help explain why there have been very few Calms and High Summoners. It would also explain all the smaller statues in the temples. Perhaps they're just those who went up against Sin and failed. I don't know. I'm looking for answers from someone who's more well versed on the world of FFX.