The correct answer is 50.
Proteins are complex biomolecules made out of amino-acids bound together with peptide bonds. They can be comprised out of thousands of several polypeptide chains with thousands of amino-acids organised in complex structures. But, some of them can contain only around 50 amino-acid and still posses all of the complex traits of an protein.
Some polypeptides contain only 3 amino-acids, but technically they are not classified as proteins due to a big functional and structural differences between a simple polypeptide chain and a protein. Proteins are made out polypeptide chains, but this is only the primary structure of an protein. the polypeptide chains in a protein are organized into complex 3D structures that give them novel traits and complexity as compared to plain polypeptide chains.