Why In a direct-sampling SDR, the analog-to-digital converter can only represent voltages up to its full-scale reference voltage. Once the RF input swings beyond that reference, the converter simply pegs at its highest (or lowest) code and the waveform is clipped, generating harmonics and intermodulation products across the passband. That reference voltage, together with the ADC's bit depth and any front-end gain, sets the receiver's maximum input level, which is why attenuation ahead of the ADC is the usual fix for overload.
Watch out The maximum count value of the converter is the digital output number that results from hitting full scale, not an input signal level you can exceed; the sample rate choices concern Nyquist and alias frequencies, which is a bandwidth issue rather than an amplitude limit.
Overload is about amplitude, not speed: the ADC's reference voltage is the ceiling. Go above Vref and you clip.