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.

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =