Astrophysics has a tradition of using the word code (as per computer code) slightly differently than the more common usage. In astrophysics it can mean a computer program or computer subprogram (i.e., procedure or subroutine) or library of subprograms. This usage appears most often for computer programs that simulate or model physical processes, e.g., radiative transfer code for radiative transfer.
Outside this astrophysical usage, "code" is commonly used as a mass noun, indicating the substance of a computer program, e.g., "I'm adding some code to my computer program this month", or "I wrote a computer program this month", but not "I wrote a code this month". In astrophysics usage, it is sometimes used in the latter manner, i.e., you can indeed speak of "a code" or "two codes".