But nowhere in the game does it ever say that there can be only one sorceress existing at a time. In fact, the tutorial says that the number of sorceresses in the world is not known. And let's not forget that Edea, followed by Rinoa who inherits her powers, exist while Adel is yet alive.

And it's also never stated that the sorceresses you fight are the ones who exist between Rinoa and Ultimecia's lifetimes. Time is being compressed, so they could be from throughout the history of the world.