Although there are only 20 amino acids, these amino acids can combine into an innumerable amount of combinations to form different and unique proteins.
In case that doesn't make sense to you, I'll provide you with an analogy. You could be provided with 20 different LEGO bricks to work with. While there may only be 20 bricks, these bricks can combine into a vast amount of different formations, structures, etc. Amino Acids work in the same way.
