It's also stated in the Black Mage quest line you can't cast the bigger/harder black magic if you don't have a soul stone or you end up dying. I think there could be more Red Mages as our trainer got taught by someone else and had a fellow student. Then we also have Geomancers who from what little we know about them are a cross of conjury and astrology. Some of the we don't see a lot of X job is probably due to having so many soul stones or teachers of the art not being around or lost to time and various calamities. I mean we've had giant earthquakes, floods, fires ice ages and at least where scholar is concerned a whole city engulfed by a magical weapon of mass destruction.