The only guidelines for class breakdown in a given settlement, either the tables in the DMG or WotC's more detailed Web Enhancement, deal with each class as an abstract. There's never been an official table/rule for how many of which specialist wizard or which god's clerics there are in a given area. This is likely because the exact breakdown is going to be too closely tied to the campaign and the settlement itself. It would take far too many tables to account for the likely variations (arcane college, city near a place of power, capital of kingdom that outlawed necromancy, etc).
That's not to say you couldn't come up with such a thing yourself and tailor it to the campaign world you're using. To that end, you'd want to look at which schools of magic are most accepted or common in a given area: most places would outlaw necromancy, regulate evocation and illusion, and make good use of divination. Generally speaking, any magic that can undermine authority or threaten the stability of a community will be outlawed or regulated and vice versa. Knowing that will tell you the rough relative balance between the specialist wizards. This doesn't account for secret cabals, cities ruled by wizards, etc., but those are important plot points and out of the scope of a random generator.