Japan fielded armies of hundreds of thousands of men, all equipped with swords. I guess if you totalled the weight on metal on them, you could cobble together a battleship. But it's not really about quantity, more quality of the ore.

And the point isn't about that. The point is that the constraint (bad ore quality) forced the Japanese to get better at metalworking. Resource constraints aren't as bad as we think, because we're used to making things without those constraints. But our descendants, having always had those constraints, will find better ways of solving them than we can think of.

Hundreds of thousands? The battles I read about maybe reached tens of thousands, and who knows how many had swords.

Even in Europe, a large part of the armies were peasants with whatever comes to hand. Their resource constraint on metals wasn't the ore, but the availability of the enormous quantities of wood required to process it.

Wikipedia says 400,000 samurai: https://en.wikipedia.org/wiki/Samurai#:~:text=In%20the%20187....

Every Samurai had at least two swords. So at least a million swords. Let's say a sword weighs 1Kg, that's 1000 tonnes of steel. So probably not enough for a battleship, but maybe a small destroyer.