A homopolymer has only one type of building block called a monomer repeated 'n' number of times. A heteropolymer has more than one type of monomer. Proteins are heteropolymers made of amino acids. While a nucleic acid like DNA or RNA is made of only 4 types of nucleotide monomers, proteins are made of

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.