From experience I have found that certain wire sizes work for most situations. Take for example 10 awg wire. I normally use this for 30 Amp mains. With 105 deg C wire the rating for a single wire outside an engine room is 60 Amps, but in an engine room it is reduced to 51 Amps, and if four to six conductors are in the bundle it is reduced to 30.6 Amps in an engine room.
I usually use 12 awg wire for 15 Amp circuits. This wire by itself outside an engine room is good for 45 Amps, in an engine room it gets reduced to 38.3 Amps, but if four to six conductors are in a bundle then the rating is reduced to 26.8 Amps in an engine room. As the bundle gets bigger the rating gets reduced even more, with 7 or more conductors the rating in an engine room is 19 Amps. So when I use 12 awg wire for 15 Amp circuits I am being cautious but it also means that I don't have to keep watching how many wires are in the bundle and I can wire the boat without constantly worrying.
In many cases I could get away with 14 awg wire for 15 amp circuits but it does fall just below the 15 amp rating with 7 or more conductors in a bundle in an engine room.
When considering how many wires are in a bundle we are counting current carrying conductors, the ground wire doesn't count. Don't forget though that as you get near the circuit breaker panel all the wires come together in one big bundle, so even on a small boat you can end up with quite a few.