Most stock alternators on a average size car are probably capable of delivering at least 60 amps. But remember, The stock alternator was only designed to handle your car's stock electrical system which includes the little stock radio, the heater fan, ignition, computer, lights, and maybe a few more accessories. Adding a big amplifier(s) that pulls 40+ amps is really adding a strain on your electrical system, mainly the alternator, but the battery suffers also.
Definitely upgrade the alternator and battery if you can afford to.