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

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.

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