If you are installing a multi-battery switch, then you are installing an isolator/combiner, all be it a manual one. Are you asking about an ACR/VSR automatic isolator/combiner?
The number of batteries and thier size, will depend on how you intend to use the system. If you mainly ride, then a 2-battery system is fine. If you intend to sit for hours anchored and play the tunes, then no, you need more battery amp hours.
You will need specific amp and speaker models in order to be able to ball-park thier average current draw.