the biggest thing you want to consider, when talking about alts and batts... is what type of batteries your going to be using (rated AH), versus the amps output... if you add too many large batteries to a single alt, you can burn the alt up...
another REALLY big thing to consider, is when the alt reaches max output... some of the lower quality alt companies out there, use changes in pulley size, to aquire higher output... but that, can mean it wont hit this output until a higher rpm.... meaning, if your parked, and at full tilt, you'll either have to rev really high, or suffer some voltage drop... just food for thought