How does adding more of a substance affect it's density?

Adding more of a substance has no effect on its density. The formula for density is:

Density = (mass) divided by (volume).

If you doubled the volume of your sample of the substance, then you also doubled the mass of the sample. So when you divide (mass) by (volume), you still get the same number. The density doesn't change.

I'm going to use molasses as an example of a substance.

The mass and volume both change when changing the amount of molasses.

However, the density does not change. This is because the mass and volume increase at the same rate/proportion!

Even though there is more molasses (mass) in test tube A, the molasses also takes up more space (volume). Therefore, the spacing between those tiny particles that make up the molasses is constant (does not change).

The size or amount of a material/substance does not affect its density.