I don't have a preference on an exact number. My view is that if you have N levels, and a bunch of them have large swaths of empty space or "filler" material, then you have too many. Contrawise, if individual levels radically change the character, you probably have too few. Those are, of course, subjective measurements.
My guess is that anything below 10 will induce class cramping, and anything above 30 will inevitably be bloated, but perhaps that's just prior experience biasing my answer.
Whatever the number, I want a minimum of a sixth of the levels, and would prefer around a third of them, in a simple core ruleset.
I feel a d20 resolution mechanic works most simply and elegantly wth a 10-level system.
Sometimes, in my crazier moments, I envision a D&D-ish game that only goes to level 10, but does it multiple times. That is, if you max out at level 10 in a given "tier", you have the option to keep all your current abilities, but redefine what new ones can do, and start over. So you go from a 10th level "standard" wizard to a 1st level "heroic" wizard who can do some nifty extra stuff with his 1st level standard wizard abilities. He gains 2nd level in the new tier, and can now do this nifty stuff with those abilities. This is effectively open-ended, only capped by how gonzo you are willing to go.
Perhaps the Burning Wheel black/gray/white distinctions are prompting this thought. With D&D, it would be pretty neat to use the various prismatic spells as the designations. You have to get all the way to "Purple" or so before you can do a "Wish" in the old school format.
