When you power our receiver using 12v, the current is ~150mA when both relays are activated, and is ~(7-12mA) during standby, depending on different hardware versions.
Usually, 12v power supply in market can supply at least 500mA, so it should be more than enough to use.
There is no maximum amperage, the maximum amperage just shows the max capability, anything less than that should work good.