why do we need to add a constant in the formula of rust

So, rust is basically formed when iron oxides are exposed to moisture and air.

It forms a coat of a reddish- or yellowish-brown flaking coating of iron oxide that is formed on iron or steel by oxidation, especially in the presence of moisture. This is called rust.

Now, the answer to your question is that the number of water molecules is actually represented by an "x" because it is not of any certain value. It simply signifies a large number of water molecules that are attached to the compound Fe2O3 forming rust.