Proteins are building blocks of life. Each protein is made of a long chain of amino acids. Amino acids are made up of an amino-functional group (-NH2) and a carboxyl group (-COOH). Although the proteins comprises of the same groups but each structure of proteins varies from another. The structure, shape, and function of a protein is determined by the sequence or series of 20 amino acids having an amino-functional group (-NH2) and a carboxyl group (-COOH), a hydrogen attached next to carboxyl group and a side chain -R which differs from each and every protein. The side chain-R can be either hydrogen, or aliphatic or aromatic or heteroatom group.
Hence the answer is 20 types of monomers.