I see this is your first post, so first things first. Welcome to RCU, and welcome to gas!

Second, I think many of us will agree, especially when talking about the size plane that will likely be found with a 10cc gasser in it's nose, that one battery will do just fine - and there's no need for it to be something huge either. The ignition module won't use much more than a servo would.

If you are buying a new battery, I would HIGHLY recommend you seriously consider a battery with LiFe chemistry. They have proven very reliable, user friendly, and they can deliver the "punch" needed to run an entire flight pack, plus ignition with no struggle.