Do you just add an “s” or do you add “es” like it would be pronounced?
CMSs
CMSes
?
Just s. And definitely no apostrophe!
Correct use of the apostrophe is a religion with me. 
Not being snarky, but sincerely – says who? I was taught explicitly that abbreviations (e.g. “TV”) were the one case where the plural was indeed formed with “apostrophe S.” There is such a thing as a TVS, and if only upper case letters are being used that apostrophe is necessary to distinguish between TVS (singular) and TV’S (plural).
Not only has every style guide I have seen said not to include an apostrophe, but why on Earth would you be capitalizing the S that makes the abbreviation plural in the first place?
Missed the edit window. I just did a search through several online style guides and they all seem to agree with what I stated above. Here is an example:
http://www.osu.edu/resources/styleguide.html
While there may be some group out there that recommends the use of apostrophes, I couldn’t find them.
What I underlined there. Surely you’ve seen signs and advertisements that use only upper case letters?
Nevertheless, it appears what I remember is not the norm nowadays.
Off-topic: isn’t it a little troubling that the style guide contains such an obvious grammatical error (‘is needed’ instead of ‘has been needed’)? The way it’s written implies that your plural abbreviations will last several decades with no apostrophes, but apostrophes will eventually need to be added. 
So how do you differentiate between Ps (multiple occurences of the letter “P”) and Ps (Static Pressure)? Or any case where adding an ‘s’ creates a different abreviation/acronym?
Until about 25 years ago, it was commonplace for style guides to say that you add an apostrophe when pluralising acronyms. I can’t find any current online cites for that, but I have read it in many places.
What? No, ‘is needed’ is absolutely correct. You use the present simple tense for rules and ‘things which are always true.’ ‘Is needed’ is the passive voice version of present simple; it’s often used for rules.
‘Has been needed’ would make it sound like you were talking about the past up to and including the present, but not necessarily into the future. It would most often be used for comparisons between the recent, continuous past and the present: ‘this development has been needed for a long time, so we’re relieved it’s finally here.’
‘Is needed’ would be fine of course, but ‘is needed for decades’ is nonsense. The closest meaning I can figure out is the one I mentioned.
Context. P’s (possessive) and P’s (plural) could also create confusion - not very likely, but neither is your example.
Taking apostrophes out of acronyms does mean that there are some potential ambiguities, but it does get rid of other potential ambiguities, and at least it means that all non-possessive plurals are now supposed to be unapostrophised, which should, theoretically, be easier for people to remember.
I see where you’ve gone wrong there: they’re not saying that apostrophes have been needed for a really long time, they’re saying that apostrophes are needed for words which are decades, written in numerical format.
Quite honestly, I don’t give a flying fuck what the style guides say.
I was taught to add an apostrophe and a lower case ‘s’. It is backed up by a certain amount of logic, looks good to me, and I’m sticking with it.
That and the Oxford comma in lists.
Heh, now I see it. I should have looked a little more closely. :smack:
I used to work for CSC, which is a large IT consulting company (90K employees). I had a copy of their technical writing style guide and it said do not use an apostrophe for plurals unless it would create an ambiguity, as in exactly this type of case.
The following is a made-up example but illustrates the point. I could not quickly come up with a real standard or product number ending in a lowercase “s” so I made one up:
When configuring the network and selecting connectors, use either five R2P’s or a single R2Ps.
(BTW is it Ps or P[sub]s[/sub]?)
Well you’ve certainly got a right to an opinion, though I don’t see the logic in using an apostrophe to make acronyms plural when it is not used to make anything else plural.
It’s hard to imagine that the difference between “multiple occurrences of the letter ‘P’” and “static pressure” wouldn’t be immediately obvious from the context.
See, there you go. “Five R2Ps” has to be plural, and “a single R2Ps” has to be singular. No need for any apostrophe.
Well, shit, since I’m making stuff up, how about this:
When configuring the network and selecting connectors, use either five R2P’s or one R2P combined with one R2Ps.
I guess you could argue it still doesn’t need the apostrophe, but without it, people are going to blink.
I see what you mean, but you could make the same stuff up where people would be momentarily confused as to whether it was a plural R2P’s or possessive singular R2P’s. You’d have to rephrase either way.
There needs to be a rule about when to use the apostrophe, and it’s simpler to say ‘don’t use an apostrophe to show pluralisation’ rather than have exceptions which lots of people would get even more wrong: ‘oh, if that’s an exception, then that might be too…’