About how many typical amino acids are found in proteins?

Most biochemists would assume that a 'protein' has enough amino acids in it to have assumed a relatively stable conformation, but there is no hard and fast lower bound for the number of amino acids that can compose a so-called protein molecule.

40 amino acids can be found in a small cytokine (a protein signaling molecule). Polypeptides are amino acid chains that are too small to be categorized as proper proteins.

There is also no hard and fast limit on the maximum size of proteins. Here, too, classification problems arise: is a protein simply folded from a single continguous chain of covalently bonded amino acids, or can it be defined as an assembly of multiple non-covalently bounded chains of amino acids?

A normal globular protein, or your garden variety mid-sized kinase, has about 1200 residues.

Proteins typically contain 20 different types of amino acids.