Let's call this "certain amount" with a variable, x, (since it's not specified in the question, I will assume that the answer shall just be based on this variable).
So every additional floor will make the building sink by x, and let's make the unit cm, so x cm.
Now we know that the maximum depression is 20 cm, and it is also
x*y, where y is the number of floors.
So we have x*y=20,
therefore the number of floors is:
20/x or
[tex] \frac{20}{x} [/tex]