9
A good-sized comet can mass more than a hundred billion tons, and in the richest comets as much as four-fifths of that mass may be water—not liquid water, of course, but ice is just as good.
Water's the first thing you want to grow crops. Old Earthie farmers used to calculate that it took about seven million tons of irrigation water to lay one inch—in modern measures, two and a half centimeters—of water on a hundred square miles ("miles"!) of farmland. The other figure to remember is that you need about ten "inches" a year to make most crops grow, even with trickle irrigation.
What that adds up to is that you could wet down nearly a million and a half square miles with one decent-sized comet, sort of.
In practice it's harder than that. You can make those sums work only if you could keep the water contained in one place—which you can't. And also if most of it didn't get blasted right back into space by the impact—which it does. And if that was all you wanted . . .
But of course the Martians wanted much more than that. They wanted water to keep, and air besides. So they needed a lot of comets, because what they wanted was to make the whole old planet bloom.
What a lucky thing it was for all of them that the Oort had literally trillions of the things.