It's to be earned, it's not rare in itself, for now it's rare to find people who can prove themselves worthy.
The Gods guide us in the process to become worthy, and later They will guide us in the Magnum Opus too.
The rest is all up to us, we have the freedom to do something or to do nothing.
It appears that under rare and perhaps extraordinary circumstances it is possible one can "accidentally" achieve a level (or several levels) of the Magnum Opus, although I am not sure to what extent the Gods may indirectly (or directly) have a hand in these cases as well, alongside eugenics factors
But I was just wondering if anyone had any information to gauge how many are currently likely to be alive, based on the rarity of the task of M.O. itself