Question

How can there be 64 codon combinations but only 20 possible amino acids?

Answer 1

Out of the 64 codons only 61 are coding while 3 are known as stop codons which do not code for any amino acid.

Now since the genetic code is degenerate this means that one amino acid can be  coded by more than codon. Except for methionine and trytophan all other amino acids are coded by more than one codon.

hope this helps you!!