The owner of my local game shop has been playing D&D longer than I have (which is saying quite a bit). He often quotes a standard that one person in ten has any real spell casting ability.
I'm not sure if that is some official figure, or what edition it might be from, but it brings up the question: How common is spell casting in your game world?
At some level, fairly common. Learning basic Wizardry is roughly equivalent in difficulty to learning a foreign language from a different langauge family as your native tongue, learning to play the piano, and learning to do basic calculus. If a person from our world could manage all three, then chances are he could manage to do a few cantrips at the least. Between 1 in 10 and 1 in 20 people could probably be taught some magical ability (depending how much effort you wanted to put into the task), though in practice no more than 1 in 100 are because of the expense and difficulty of learning. Most flame out at a relatively low level of ability and never obtain more than 1st level spells. About 1 in 1000 people go on to become successful practicing magicians, usually obtaining the ability to cast a handful of 3rd level spells. Perhaps as few as one in 100,000 obtain the 5th circle and the rank of Wizard (9th level).
Very high level spellcasting is much more difficult than that. It requires skills roughly equivalent to being able to speak multiple languages from several language families, being able to understand deep theoretical quantum mechanics, and to able to play the piano as a virtuoso. There are only a few people in the world at any given time that can manage such a skill set, and often a generation might pass before a new master arrives on the scene. Were it not for the fact that many of these most powerful individuals find means of prolonging their mortal lives, no more than one or two would exist in the whole world.
Sorcery is something you are born with and which cannot be taught. Sorcerers in my world are generally classified as monsters, and this is often warranted (though some would say its a self-fulfilling prophesy). The percentage of sorcerers out there is supressed by the fact that sorcerers are often executed or murdered (depending on your perspective) at a very young age when their talents first manifest (often as not as toddlers!), and are generally subject to death in many parts of the world whenever their secret is discovered. It's a lucky sorcerer that doesn't manifest his power until adolescence when he has at least something like the emotional and intellectual ability to control it; unfortunately these are often the weaker and more limited sort. Perhaps 1 in 1000 non-fey free people manifest sorcerous ability - fewer than one in 10 though survive to adulthood. Fewer still have it in their blood to manifest truly great power, and many never try for fear of revealing themselves to a hostile world. Still, there is some safety in power and at high levels sorcerers are roughly as common as wizards. Sorcery among fey is of course far more common, being perhaps 1 in 10 individuals and is generally accepted as normal by fey.
Bards vary from exceptionally rare to fairly common depending on where you are. In most civilized lands, bardic magic is considered proscribed magic and is classified alongside necromancy, shamanism, diabolism, sorcery, and most enchantment magic as dark arts. Indeed, there are some people that would consider it worse, because its believed to be the last surviving vestige of Art Magic - the worst of all crimes against the universe. In such cases, bards are probably rarer than sorcerers. However, among the Orine tribes in particular, bardic magic has a very strong tradition and no travelling band is without one. The overall numbers here are probably in the range of 1 in 500, but in most human lands its closer to 1 in 10,000.
Clerics are good deal more common than either arcane caster, as the number of clerics depends less on native ability than it does divine ordination. Usually, between 1 in 80 and 1 in 100 persons have some degree of clerical ability and can channel some sort of divine power. In some particularly theocratic and pious areas, it may be as much as 1 in 40. Not only are clerics more numerous, but relative to their number there are somewhat more higher level clerics than Wizards.
My game world doesn't have druids, but it does have shamans. In most areas, Shamans are slightly less common than Clerics. Shamanism requires a somewhat less rare skillset to learn compared to Wizardry, and the traditions are often passed on in secret from parent to child (often but not always through the female line). Attainment of more than 3rd level spells is quite rare though, and very high level shamans are if anything rarer than high level wizards. Partly this is because its practioners seldom attempt or desire to obtain high levels of power, partly this is because the knowledge of how to do so is rare and jealously gaurded, and partly this is because in most areas shamanism is officially illegal and its practicioners subject to being burned at the stake (though this law is often ignored in rural areas save when its deemed necessary). Shamans and clerics are in fairly direct competition with each other, so where one dominates the other suffers. In some wilderness areas, there are virtually no clerics and some shamanistic knowledge will be found in as many as 1 in 40 individuals.
