Well that depends on time and duty cycle. So the truthful answer would be impossible to give if this were mission critical power delivery. Since you have the option of simply turning it down when you see the voltage drop, it's really not a problem. You will never draw 500 amps without letting out the magic smoke. The ah rating means this (how much current can be drawn from the battery for 1 hour before dropping to 10.5v) Now, if you get to 10.5v your amp will certainly blow. We're looking more like > 12v. So, this cuts the current you can draw for an hour and keep your amp alive in half... or the time. So, you have a 55 ah and 500 amps of draw. You still need 445ah worth of batts to keep your amp alive for 1 hour of playtime using batts alone (assuming there is no alt and it's a constant tone.)
If it were ME, I would do a HO alt (at least 160 amps at idle) and 4 group 31 batts in the back. Remember, the batteries cannot recharge if they are constantly discharging. They are not capacitors and able to almost instantly recharge between bass notes. They take several hours to recharge after discharging several hundred amps. No matter how many batteries you add, you're only buying yourself playtime. Not higher voltage and not indefinite playtime.
It ALWAYS makes your amp perform more efficiently and reliably to upgrade your alt. After you see how many batts you need to just keep your amp alive, you should start to realize that an alt is NOT more expensive than a bunch of batts. Especially once you factor in amplifier repair costs.