The various articles relating to the classes are of special interest to me, but lately I've been asking myself the question, "how many classes is too many classes?"
I wrote a few years back that if your class based system had more than 20 classes - regardless of whether you called them 'base classes' or 'advanced classes' - it pointed to fundamental flaws existing in your design.
Some examples:
a) Encoding personalities in to your classes such that any two members of the class will tend to be fundamentally similar.
b) Encoding moral philosophies in to your classes: ex, you've got a class for champions of lawful good, so now you need parallel classes for chaotic good, chaotic evil, lawful evil, etc.
c) Making non-archetypal trivia mandatory in your class design. ex, your clerics all must use non-bladed weapons.
d) Having more than one class that does the exact same thing but with different mechanics, ex: your sorcerer class is so inflexible that you also have to have a Psion class, or your wizard class is so inflexible that you need a necromancer class. Or maybe more to the point you have fighter, knight, samurai, chevalier, warmain, warlord, duelist, monk, kensai, and two dozen other classes which have as their core concept 'excels in some aspect of martial combat'.
e) Encoding cultural artifacts into your classes when the culture is not universal to your entire setting, ex: you have both a samurai and a chevalier class when really all you need is someway to build a noble horseman.
f) Siloing very generic abilities into particular non-generic classes so that any concept that involves that generic ability must in some way involve the class, ex: the ranger's tracking ability or the rogues climbing ability in 1e.
The problem with having lots of and lots of classes is that not only is it unwieldy from a rules perspective, but it tends to actually reduce player choice rather than increase it. Regardless of what concept the player has in mind, there is some fiddly class out there that comes close to that concept but in fact shoe horns the character into a rigid development path and forces the player to carry or offload a bunch of baggage whether thematic or mechanical that is unrelated to his concept. Trying to solve this problem by create a nearly infinite array of highly specialized classes suggests you'd be better off dumbing classes for a different character building methodology. However, the problem can be avoided almost entirely by avoiding bad design in your classes in the first place.
True classes are configurable archetypes that collectively cover all concept possible in the setting. There isn't a fixed set of classes that do this or a fixed division of roles that accomplish it, but regardless of setting the set of all classes is probably a number between 3 and 20.