Answer
Though the number of amino acids is limited to 20, they can be combined in any combination, and in varying lengths, leading to an infinite number of possibilities.
Their sequencing is controlled by the DNA nucleotides contained within the genes of a cell.
Work Step by Step
If we were able to combine the letters A, B, and C, in any combination under the length of three and without repeating any letters, we could get 15 unique combinations (A, AB, AC, B, BA, BC, C, CA, CB, ABC, ACB, BAC, BCA, CAB, CBA).
Imagine if we were to have 20 letters, and could repeat letters, and build the chain as long as we wanted (for example, the protein Titin has 34,500 amino acids). Thus, it's easy to be able to create an infinite number of unique combinations.
