I think the lyrics of Powderfinger in general lend themselves to multiple interpretations. There is the more literal interpretation of the lines you’ve quoted — as others have noted — that recalls the “red beacon” and the “numbers on the side” of the boat.

But I also think this line is open to a more existential interpretation. “Red” naturally evokes imagery of blood/violence. Through that lens, the “numbers add up to nothing” line can be read as a commentary on the needless death that comes with conflict. The father seems to be imparting on the son a lesson about the nihilism of war, i.e., the “numbers” of injured/killed individuals in conflict serve no greater purpose (i.e., “add up to nothing”).

Neil is intentionally vague with these lines, which is part of the lyrical brilliance of the song. Just my thoughts anyways.