the capacitance of a conductor does not depend on the nature of its material and the amount of charge existing on the conductor. why?

When a conductor is given some charge it is raised to some potential. As more and more charge is given, its potential increases accordingly. If charge q is given to the conductor so that its potential increases by an amount V then it is found that q∝V or q = CV.

where the constant of proportionality C is the capacitance of the conductor. Its value depends on the shape and dimensions of the conductor. The capacitance of the conductor does not depend on the material of the conductor but depends upon the nature of the medium.

So C is just a constant in the equation of charge and does not depend on it.