4^3 = 64
There are 64 ways to arrange the four bases (Adenine, Thymine, Cytosine, and Guanine) into triplets (include repeats of bases as well, for e.g. AAT).
Just in case you are confused by my use of Thymine above, one thing to note is that Thymine will be replaced with Uracil in RNA.
Hope this helps! :)