It all depends on the amplifier tht U have in mind.
Stereo or mono and Power requirements to the desired load.
All in all a stereo one will need twice the current rating that is needed for the same mono amp.
Current and voltage depends on the amp power rating.
Voltage supplied from the PSU should typically 3 to 5V higher than the output voltage swing to prevent clipping at full output.
Clipping will damage any amp plus speakers. And will also result in severe distortion.