As far as I know the idea of critical bands is to model auditory filters.
Looking at the basilar membrane only: As the basilar membrane is a continuum, with a continuous variation of mass, damping and stiffness, there is a continuous variation of the natural frequency along the basilar membrane.
Therefore, the basilar membrane function can be modelled by an infinite number of (non-symmetric, and non-linear) pass-band filters, e.g. critical band filters.
In practice however this would not be very practical.
The critical band model from Fletcher and Munson, as well as the model of Zwicker use 24 discrete critical bands as an approximation.
The Cambridge model (Galsberg and Moore) approximates with 48 discrete critical bands, I think.
Best
Stephan