How many different mrna codes correspond to methionine?

Only one genetic code corresponds to methionine.

Methionine is encoded by a single codon (AUG) in the standard genetic code. The methionine codon is also the most common start codon (message for a ribosome to initiate protein translation from mRNA). As a result, methionine is often incorporated into the N-terminal of proteins in eukaryotes during translation.