Potentially, using the genetic code as it exists, nature could have used
63 amino acids. why do you think it only uses 20?

Genetic code comprises 64 codons, out of which 61 codes for 20 amino acids, while the remaining three codons do not code for any amino acids. In nature, only twenty standard amino acids prevail.  

A standard amino acid is considered as an amino acid that possesses an alpha-carboxyl and alpha-amino acid, and which are found in the proteins. Though there are some of the compounds that possess alpha carboxyl and alpha-amino group but are not regarded as standard amino acids, as they do not take place in proteins like the standard one does, and these are termed as non-standard amino acids.  

The frequency of these non-standard amino acids in the proteins is less and they do not comprise all the characteristics demonstrated by the standard amino acids. The genetic code is degenerative and because of this, a single amino acid is encrypted by more than one codon.  

The non-standard amino acids do not exhibit the corresponding amino acids, which can code for them and they are not mediated by the t-RNA's to the location of translation, as there is no corresponding tRNAs for them. Thus, genetic code utilizes only the twenty standard amino acids.