Well, seeing that a switching power supply is really a sampled-data system, the maximum bandwidth is limited to 1/2 the switching frequency (Nyquist's theorem).
However, in the real world, the bandwidth should be limited to about 1/4 to 1/5 of the switching frequency.
But that is not a rule. And this is just an upper limit, it does not HAVE to be that high. Bandwidth can be only a few kHz. You make the bandwidth just high enough to obtain the transient response you need, while ensuring stability under all conditions of input voltage output load and additional output capacitance, which can be added to the load.
The switching frequency can be increased without a theoretical limit, current designs run into the MHz range. What limits the switching frequency is speed of the transistors, the losses in the magnetics and windings and the difficulty to drive the transistors (capacitive load), which ultimately translate into higher losses. Also, do not forget the EMI issues, which can also paly a role in choosing the switching frequency.