As many as it needs to let me represent my character concept properly in mechanical terms, IMO. If one class does the job, all the better. But if it cannot, then I see no reason to impose arbitrary limitations on how far I can multiclass.
Classes are a metagame concept to begin with. Fighters do not even need to have 1 lv in the fighter class. There is no shame in multiclassing between 2-3 base classes and 3-4 prcs if it will allow me to achieve my intended result, nor is there anything to feel proud about for staying in 1 base class all the way from lv1 to lv20.
There is such a thing as too many crappy classes (such as the samurai) or classes which are pointless past a few lvs (eg: marshal, swashbuckler prior to daring outlaw). But that is more a case of poor design rather than a case of class glut.

Classes are a metagame concept to begin with. Fighters do not even need to have 1 lv in the fighter class. There is no shame in multiclassing between 2-3 base classes and 3-4 prcs if it will allow me to achieve my intended result, nor is there anything to feel proud about for staying in 1 base class all the way from lv1 to lv20.
There is such a thing as too many crappy classes (such as the samurai) or classes which are pointless past a few lvs (eg: marshal, swashbuckler prior to daring outlaw). But that is more a case of poor design rather than a case of class glut.