It all depends on how "thick" your shellac is, and what you're going to use to apply it.
Shellac is mixed in "pound cuts", which is a measure of how many pounds of shellac flakes are dissolved in a gallon of denatured alcohol. Obviously, the more shellac per gallon, the thicker each coat will be. If you're mixing it yourself (which will result in the most durable shellac), you have control over the pound cut. If buying the Zinsser pre-mixed shellacs, you have to deal with whatever pound cut they give you. I think their waxed shellac is a 3 pound cut, and the dewaxed stuff is 2 pound, but I don't know for sure...I always mix my own (and if you want tips for doing this, let us know).
In any case, if applying it with a brush, I'd go with a 2 pound cut. If applying it with some sort of floor applicator, thin it out a bit (maybe 1-1.5 pound). Shellac dries so fast that you an apply numerous coats one after another.
FWIW, I'm currently finishing a cabinet I built with 1 pound shellac, and it took about 8 coats (with two light sandings mixed in) to get the build I wanted. For a floor, I'd put more coats on.