How many amino acids are there?

According to the ASU School of Life Sciences, 21 amino acids are used by the human body to build all the protein it needs. The exact number of universal amino acids may differ depending on the source, as some give 20 or 22 as alternative answers.

As the building blocks of protein, amino acids can be configured in countless ways, forming innumerable variants from the standard number. Essential amino acids, or those that the body cannot synthesize on its own, include nine of the universal amino acids. The body, therefore, must attain these essential amino acids from what it ingests and metabolizes.