Note that, for what it's worth, fire pumps are generally rated for their capacity when drafting from a static water supply (think, pond, lake, river, etc). Basically all modern fire pumps can easily exceed their rated capacity by a pretty good margin when pumping from a pressurized source, but then you're back to your point of "do you have a source that can supply that?" Still, there are ways. In my firefighting days we had some hydrants in our district (the ones on the big 30" main that ran right down the middle of the county in particular) that could individually supply 2000gpm. And nothing says you are restricted to using one hydrant! There are also all sorts of complex water supply evolutions one can run, involving relay pumping with multiple engines, drafting and using hydrants, etc.