Given long enough, all the water in the world.
Given enough time the entire ocean could be pumped through a half-inch pipe.
Gray cast iron is around 118 grams per cubic inch.
A lot
Pure (or distilled) water does not contain iron. If the water has travelled through an iron pipe, it will dissolve some iron. Well water or water from many other sources contains some iron because the soil contains some iron compounds, which can dissolve. Sea water contains iron. But the actual amount varies quite a bit.
Depends on the diameter of the pipe and the speed at which the water travels through it.
So we can see how much the water is up to so if we know if it is high or low approximately how much it is compare to the steam iron.
300 gallons per minute
As much as you want. But a larger quantity will take longer to pass through the hose.
question cannot be answered because it depends on the water pressure
None. A square inch has no volume.
orange tacos
Depends on what's flowing (gas or water or something else) and under how much pressure.