How to pluralize the following initialism [duplicate]

RFS is an initialism. In other words, you sound out the individual letters that make up its short form.

(Unlike like NAFTA and NATO which are acronyms, and are pronounced as words.)

It used to be that the plural form of an initialism (or acronym) had an added 's. Currently, most style guides say to drop the apostrophe and simply add the s.

Regardless of the past use of 's or the current use of just s, this is done whether the initialism already stands for something plural or not. It's not what it stands for that is being "counted," it's the initialism itself.

Therefore, the plural of RFS is most commonly recognized today as RFSs.

Oxford Dictionaries:

When you are forming the plural of an initialism, you do not need to use
an apostrophe, for example:

MPs
   e.g. MPs voted against the bill.
CDs
   e.g. I bought some new CDs today.

Note that the possessive form of initialisms is formed in the usual way,
with an apostrophe + s

The Chicago Manual of Style Q&A:

What is the correct way to make acronyms or initialisms plural? 

If you can stop thinking of the spelled-out meaning of the acronym and
just treat the acronym itself as a word with its own meaning, you should
be able to add that little s without fretting. 

Public Works and Government Services Canada:

Add an s, but not an apostrophe, to form the plural of most
abbreviations.

(Note that the page provided by the Public Works and Government Services Canada link discusses a number of specific exceptions—however, the answer I quoted here applies to RFS.)