Does the amount of a particular matter affect its density?

Answer 1

The simple answer to this would be no. Density is the mass per volume of a substance so if you were to add either one of these quantities the other would have to rise by a proportional amount and the density would remain constant.

However this become untrue when the amount of matter you are dealing with become on the order of a planetary mass. This much matter will produce a gravitational field which will cause pressures to result that will be enough to invoke a phase change in the portions from the center of the mass on outwards. In this case the density will change with the change in phase.