How are most macromolecules synthesized from a small collection of simple molecules?

Most macromolecules are polymers of simple molecules (monomers). There is not a great variety of simple molecules, but their structure determines the macromolecule. For example, proteins are macromolecules synthesized from amino acids. There are only 20 amino acids and a great number of proteins made of different combinations of those 20 amino acids.