batteries and switches

[EBEACH]

Should I put in 1 or 2 batteries for my 20’ sceptre? I would really like to only have one, but would like your opinions as to why a may need two, or is one really enough.

Secondly, Do I need a battery shut off switch? Is this just a safety device if something goes wrong like an electrical fire, or does it isolate stray currents or what ever when not in use? The boat will be on a mooring.

Thanks.