Well, the car amp was designed to run off 12V. Estimate (or, better, measure) how many watts you're *really* going to run it at, figure out how long you need it to run, then look for a 12V battery which can deliver at least that many watt-hours.... Probably a bit more, since battery voltage tends to dip before the end. Note that you don't need that much wattage to get more-than-loud-enough, given tolerably efficient speakers and assuming your goal isn't to deafen people.
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You need to know the the current it uses, the voltage it uses and for how long you want it to run before you can find out what batteries you should use.