You could wet down nearly a million and a half square miles with one decent-sized comet. But 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 planet bloom.

What a lucky thing it was for them that the Oort had literally trillions of the things.