I have a 50 amp power supply with an amp meter. With ignition on and everything settled down it draws about 15-20 amps so I would expect 20 amp to be the minimum to maintain a constant voltage. It will draw 40-45 amps when first connected and ignition first turned on and settles down in about 5 minutes or so. It will probably take longer to settle with a power supply with less total amperage than mine. Make sure it's regulated or it'll probably smoke itself.
As ******* says, you don't really need one for coding; only for programming where a single session can take hours. You'll want a constant voltage that whole time.