Assuming 1d100 stat generation, and that 90 prime is a requirement 10% of any population will be able to be a caster of a given realm. 1% will be potential hybrids. . .0.1% will be potential tri realms.
Of course, that assumes if you have a 90 EM you're Essence. . .not a 40 In, 90 Em of the realm of channeling. (if realm is not fixed to high stat, it cuts the odds to 1/3 the above). But assuming realm and stat run in synch. . .
So in Ancient Rome. . .100,000 potential magicians, 10,000 potential Sorcerers or Mystics, and 1,000 potential archmages. . . .just in the city itself.
If you assume you just need a 90 potential, that might double or triple. (profession is fixed in adolescence, so likely not)
Of course, how many of those are uneducated, of the wrong social strata, or in a religion that frowns on magic and burns witches? They might never learn a spell.
If the empire actively tested and trained casters regardless of caste or background, that number might be close to actual, depends how diligent and efficient they are.
What if the imperial authority and/or church hunt down and kill casters?
Also, as the seat of a major empire, it might draw caster potentials out of the whole empire, draw them in to the capitol, so that number could be much much higher. . .or the capitol might send out their hordes of mages to hold the imperial frontier, so most of the casters in residence would be non combative types, trainees, or central management, and lower in overall number.
What if casters are required to join a celibate order, or otherwise prevented from reproducing? What if only casters are allowed to have children? Could skew the odds in a remarkably short turnover of generations if the capacity is at all genetically based.
A lot of social and economic variables in play that can radically skew results.
Then again, I'm not a big fan of prime requisite stats, but that's the best way of measuring proportions of caster possibles in a given population.
But, too many variable factors under GM control to really state there's one obvious generic answer that fits all worlds, even just using the base core rules basic stuff like "How many people in the kingdom?" determine answers. . . .in a nation of 50,000 people, the odds above say that at most there are 5000 magicians, 500 essence hybrids, and 50 archmages. . .who might actually be farmers or some other more common profession, even though they have the capacity to be a caster, they never encounter or learn magic. . .in a low population world, casters might be exceedingly rare. Unlike Channelers, magicians appear to need to be taught, rather than made. . .so you need for one of 5000 random people in a kingdom to encounter someone who can teach them, early enough to where they can learn (i.e. in adolescence). . .what are the odds?