Question: How many standard amino acids are there?
About the article "Biochemistry", section "Proteins".

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Technically, any organic compound with an amine (-NH2) and a carboxylic acid (-COOH) functional group is an amino acid.
The proteinogenic amino acids are small subset of this group that possess central carbon atom (α- or 2-) bearing an amino group, a carboxyl group, a side chain and an α-hydrogen levo conformation, with the exception of glycine, which is achiral, and proline, whose amine group is a secondary amine and is consequently frequently referred to as an imino acid for traditional reasons, albeit not an imino.
The genetic code encodes 20 standard amino acids.
However, there are three extra proteinogenic amino acids: selenocysteine, pyrrolysine and "N"-formylmethionine.
The former two do not have a dedicated codon, but are added in place of a stop codon when a specific sequence is present, UGA codon and SECIS element for selenocysteine, UAG PYLIS downstream sequence for pyrrolysine.
Formylmethionine is an amino acid encoded by the start codon AUG in bacteria, mitochondria and chloroplasts, but is often removed posttranslationaly.
There are various groups of amino acids:  20 standard amino acids  23 proteinogenic amino acids  over 80 amino acids created abiotically in high concentrations  about 900 are produced by natural pathways  over 118 engineered amino acids have been placed into protein These groups overlap, but are not identical.
All 23 proteinogenic amino acids are biosynthesised by organisms, but not all of them are abiotic (found in prebiotic experiments and meteorites), such as histidine.
Many amino acids, such as ornithine, are metabolic intermediates produced biotically, but not coded.
Others are only metabolic intermediates, such as citrulline.
Others are solely found in abiotic mixes, such as α-methylnorvaline.
Over 30 unnatural amino acids have been translationally inserted into protein in engineered systems, yet are not biosynthetic. 
In meteorites and in prebiotic experiments (e.g.
Miller–Urey experiment) many more amino acids than the twenty standard amino acids are found, several of which at higher concentrations that the standard ones: it has been conjectured that if amino acid based life were to arise in parallel elsewhere in the universe, no more than 75% of the amino acids would be in common.
The most notable anomaly is the lack of aminobutyric acid. 
A protein is a polymer that is composed from amino acids that are linked by peptide bonds.
There are more than 300 amino acids found in nature of which only twenty, known as the standard amino acids, are the building blocks for protein.
Only green plants and most microbes are able to synthesize all of the 20 standard amino acids that are needed by all living species.
Mammals can only synthesize ten of the twenty standard amino acids.
The other amino acids, valine, methionine, leucine, isoleucine, phenylalanine, lysine, threonine and tryptophan for adults and histidine, and arginine for babies are obtained through diet. 
Aside from the 23 proteinogenic amino acids, there are many other amino acids that are called "non-proteinogenic" or "non-standard" (although three of the proteinogenic amino acids are also called "non-standard", not being encoded simply by the universal genetic code but requiring additional contextual information).
Those either are not found in proteins (for example carnitine, GABA) or are not produced directly and in isolation by standard cellular machinery (for example, hydroxyproline and selenomethionine).
Non-proteinogenic amino acids that are found in proteins are formed by post-translational modification, which is modification after translation during protein synthesis.
These modifications are often essential for the function or regulation of a protein; for example, the carboxylation of glutamate allows for better binding of calcium cations, and the hydroxylation of proline is critical for maintaining connective tissues.
Another example is the formation of hypusine in the translation initiation factor EIF5A, through modification of a lysine residue.
Such modifications can also determine the localization of the protein, e.g., the addition of long hydrophobic groups can cause a protein to bind to a phospholipid membrane.
Some nonstandard amino acids are not found in proteins.
Examples include lanthionine, 2-aminoisobutyric acid, dehydroalanine, and the neurotransmitter gamma-aminobutyric acid.
Nonstandard amino acids often occur as intermediates in the metabolic pathways for standard amino acids – for example, ornithine and citrulline occur in the urea cycle, part of amino acid catabolism (see below).
A rare exception to the dominance of α-amino acids in biology is the β-amino acid beta alanine (3-aminopropanoic acid), which is used in plants and microorganisms in the synthesis of pantothenic acid (vitamin B5), a component of coenzyme A. 
Proteins are very large molecules – macro-biopolymers – made from monomers called "amino acids".
There are 20 standard amino acids, each containing a carboxyl group, an amino group, and a side-chain (known as an "R" group).
The "R" group is what makes each amino acid different, and the properties of the side-chains greatly influence the overall three-dimensional conformation of a protein.
When amino acids combine, they form a special bond called a peptide bond through dehydration synthesis, and become a polypeptide, or protein.
In order to determine whether two proteins are related, or in other words to decide whether they are homologous or not, scientists use sequence-comparison methods.
Methods like Sequence Alignments and Structural Alignments are powerful tools that help scientists identify homologies between related molecules.
The relevance of finding homologies among proteins goes beyond forming an evolutionary pattern of protein families.
By finding how similar two protein sequences are, we acquire knowledge about their structure and therefore their function.