Below, a view of a sinusoidal signal at 5 differents frequencies (1000Hz to 5000Hz)

The bandwith is 10 times less than the sample rate or 2KHz in this case.
This comes about because you need at least 10 samples per cycle of the signal you are measuring to get any decent waveform to actually view.

If you measure a 5KHz sine wave with this 20KS scope, you will only get 4 samples of the waveform. (20000/5000)
So the sine wave will look like a triangle wave!