100 milligrams equals how many milliliters?

The question, as stated, cannot be answered sensibly. A millilitre is a measure of volume, with dimensions [L3]. A milligram is a measure of mass, with dimensions [M]. The two measure different things and elementary dimensional analysis teaches that you cannot convert between measures with different dimensions such as these without additional information.

As a simple mental exercise consider a millilitre of air and of water. They will have very different masses.