If you have a substance, would it have the same density no matter what mass of the substance you have?

Assuming all other conditions equal, then, yes, the density would not change regardless of the mass of the substance. This is because the density is a ratio of the mass substance to its volume. If you increase the mass, the volume displaced would also be greater. So, it would just adjust making the density constant. This is applicable if it is at the same temperature and pressure conditions.

Because a model would make the investigation easier to understand