I agree on using stranded, tinned, marine grade wire and cable. Running just one wire in a conduit would be silly. You can find the capacity (number and size of wires or cable for the conduit you are using) on the Internet. Since you're going to insulate this, if there's any question, use fewer, rather than more wires per conduit because the insulation will retain heat in the conduits.
Consider an extra run of empty conduit for future use. It's cheap if you install it before you spray in the insulation, not if you need it later.
As FF posted, do not mix high and low voltage circuits in the same conduit (or junction box).
I don't know if you've considered it, but type "ENT" flexible non metalic conduit is a lot easier to work with in some cases than rigid conduit and fittings, especially on a project like a boat.
You appear to be new at electrical work so you might want to run your plans by a marine electrician or have him/her inspect your work before you insulate and cover everything up.
This is a good place to buy your cable and connectors: