Maybe each savior is unique and like there is a savior for each realm / kingdom or whatever
I hope not! Nor do I want the role of Savior to be passed down through death. The only way I can imagine the idea of multiple Saviors working is if a Savior is created every time some powerful evil magic (like a curse) is performed. It could be a magical law that such dark magic always results in a Savior, kind of like the key to a code being contained within the code itself. The destruct mechanism for the curse would come into existence along with the curse itself.
But honestly, I’m quite wary about ‘delving into Savior mythology’. They’ve already confused things a bit by stating that it’s the Savior’s job to give everyone their happy ending. To me, putting this responsibility on the shoulders of one person is ridiculous. What if some people don’t want a happy ending? What if some don’t deserve it? What is a happy ending anyway? This is a job for someone omnipotent, which Saviors clearly aren’t. It makes much more sense for a Savior to give everyone a chance to get their happy ending. This would fit in with Emma lifting the first curse, as the whole point of that was to keep everyone from what they loved. A Savior should restore free will/memories or whatever, but each person should always be resonsible for their own happy ending.