How many amino acids are known to exist in nature?

The correct understanding of the number of amino acids existing in nature is that there are 20 standard amino acids that are directly encoded by genetic material and are used in the synthesis of proteins. These 20 amino acids make up the primary building blocks for proteins in living organisms, and they are classified based on their properties, such as polar or nonpolar, charged or uncharged.

While it is true that there are over 500 amino acids that have been identified in various organisms, many of these are non-proteinogenic, meaning they are not incorporated into proteins during translation. Furthermore, there are about 100 amino acids that have been recognized in specific contexts, but again, most of these do not play a direct role in the synthesis of proteins as the standard amino acids do.

The number 64 relates to the genetic code, specifically to the codons (triplet sequences of nucleotides) present in DNA and RNA that correspond to these amino acids, but it does not represent the actual count of distinct amino acids.

Due to all of these factors, the acknowledgement of 20 standard amino acids is the most scientifically accurate response.