The Chicago Manual of Style has an interesting way to address this: They omit the apostrophe, unless there are periods in the abbreviation.
So this would give you ATMs , or alternately A.T.M.'s .
( A.T.M.s looks weird.)
chicagomanualofstyle.org, "Plurals"
This page indicates that acronyms ending in the letter "S" get an apostrophe, something I've seen before, but can't find in a general reference.
So one would write ATMs and SOS's .
This page on the North Carolina State University website references AP's rule as being to always use an apostrophe.
The 2009 AP Stylebook's "plurals" entry has no section on acronyms, but mentions "VIPs", I can't find anything addressing how to specifically pluralize acronyms.
(The "abbreviations and acronyms" section is also of no help.)
Personally, I omit using apostrophes unless I can't avoid it.
I do use them when talking about single letters or where it would avoid confusion.
(For example, SOs for "Significant Others" looks like an incorrectly capitalized SOS.)
To paraphrase Carol Fisher Saller , the clearer usage is the correct one.
english.stackexchange.com/users/161/Goodbye Stack Exchange
I agree with Wikipedia , wordreference and CMOS - acronyms and initialisms are "regular" nouns; plurals are formed by adding "s".
Checking Google Books for actual usage in a relatively "contentious" case, I searched for: "OSs" unix windows linux 3120 written instances
"OSes" unix windows linux 1060 instances "OS's" unix windows linux 520 instances "Simpler" cases such as CDs vs CD's are even more decisive (over 10:1 in favour of the former).
english.stackexchange.com/users/2637/FumbleFingers
The first is the correct usage, in my view.
The third may be quite acceptable however, since the M in ATM could equally stand for 'machine' or 'machines', though I think pluralising the actual acronym is much clearer in speech.
In any case, never use an apostrophe.
's should only be appended to a word to create the posessive form ("of ..."), never for plurals.
english.stackexchange.com/users/184/Noldorin
Oxford Dictionary [e.g. SOS, noun (plural SOSs) ] and The Economist [e.g. Are ATMs stealing jobs? ]
both go for the first option.
english.stackexchange.com/users/128/Evan
Since this is a question about acronyms, and the Federal Government's bureaucracy is notorious for using acronyms, I decided to look up the answer in the United States Government Printing Office (GPO) Style Manual (2000).
Rule 8.11 of the GPO Style Manual states: "
While an apostrophe is used to indicate possession and contractions, it is not generally necessary to use an apostrophe to show the plural form of most acronyms, initialisms, or abbreviations, except where clarity and sense demand such inclusion."
As examples, the rule suggests: OKsABCsRIFsYWCAs
The rule does not show an exception for an acronym, but does refer to one case I found interesting -- the "Oakland A's" needs an apostrophe because otherwise it would be the "Oakland As."
From that I would assume that if the addition of an s to an acronym would appear to give the acronym a different meaning, then an apostrophe would be in order.
But since acronyms are capitalized letters, the addition of a small s should not make a difference, except where (for some reason) one is writing in a format that is all capitals -- such as the format that military and diplomatic messages were sent until very recently.
english.stackexchange.com/users/38718/Bruce James
I vote for the first, "ATMs".
The second is just wrong (apostrophe is not used for plurals, ever).
This is because ATM is a defined term for an AT machine , and using it as plural "automatic teller machines" would be a redefinition of a common abbreviation, which one should not be trigger-happy about.
The third, however, does not solve the real need to say there's more than one.
It is though clear from the sentence, but might not be so in a general case.
english.stackexchange.com/users/188/Pavel Radzivilovsky
Using 's to pluralize something is called a "Greengrocer's apostrophe".
I think the battle against the Greengrocer's apostrophe is one we're bound to lose - even if grammar of the general population improved, we'd still occasionally be facing nouns which have a mixture of upper and lower case, for which adding an s by itself at the end would be confusing.
english.stackexchange.com/users/1420/Andrew Grimm
Either of the first two is acceptable, but I would recommend the first as the apostrophe isn't needed to convey your meaning, and as such is not required.
The third is just wrong since it creates an awkward sentence that is hard to say and discomforting to read.
Most acronyms, including ATM, have a well-defined and commonly accepted meaning, which very rarely includes the pluralization.
Avoid the ambiguity and include the s.
english.stackexchange.com/users/171/jeffamaphone
Just a small addition to the subject and one that is probably as much related to typography as it is to grammar...
There is an issue with all-caps.
For example, if we were: Talking about ATMs.
Then the apostrophe is out.
But, if we were: TALKING ABOUT ATM'S.
Then the case for using one is much stronger as it serves to differentiate A utomated T eller M achine from, say, A utomated T eller M achine S oftware Of course, that doesn't address the question of why you'd be using all-caps in the first place...
english.stackexchange.com/users/54223/PerryW
For example, if I wanted to write the equivalent of   There are many automated teller machines in this city.   Would it be   There are many ATMs in this city.   or   There are many ATM's in this city. (could get confused with possessive form or contraction).   or just   There are many ATM in this city. (assuming the final s is included in Machines represented by M).   Maybe something else?
What is the correct way to pluralize an acronym / initialism?
english.stackexchange.com
<grammatical-number><orthography><apostrophe><acronyms><initialisms>
english.stackexchange.com/questions/503
english.stackexchange.com/users/179/JohnFx
The answer to this question will be subjective, but the most popular way of pluralising an acronym is probably to use an s with no apostrophe. It is generally agreed that apostrophes should only be used to indicate possession. An apostrophe is used to indicate possession and contractions, but not to show the plural form of most acronyms, initialisms, or abbreviations, except perhaps for acronyms separated by periods or ending in S.
The answer to this question will be subjective, but the most popular way of pluralising an acronym is probably to use an s with no apostrophe. It is generally agreed that apostrophes should only be used to indicate possession. The general consensus is that you should never use an apostrophe.  But if the addition of an s to an acronym would appear to give the acronym a different meaning, then an apostrophe would be in order.  Since acronyms are capitalized letters, the addition of a small s should not make a difference and it should be clearly understood. Therefore, acronyms and initialisms are "regular" nouns;  plurals are formed by adding "s".  
In this example "ATMs" would be the more widely accepted correct usage/The apostrophe should be used for possessive forms, not for plurals. Wikipedia ,  wordreference  and  CMOS  maintain that acronyms are "regular" nouns and  their plurals are formed by adding "s". It is generally agreed that you do not use an apostrophe to pluralise any word or acronym. However, there will be some who defend the use of apostrophes when pluralising acronyms if such use helps correct understanding.
Only use an apostrophe if the abbreviation has periods between each letter or ends in S. Acronyms and initialisms are "regular" nouns;  plurals are formed by adding "s".   Never use an apostrophe. An apostrophe is used to indicate possession and contractions, but not to show the plural form of most acronyms, initialisms, or abbreviations. Go for the first option - apostrophes are never used for plurals The apostrophe is technically incorrect but it is still useful in some cases and so is probably here to stay. Both are OK but the first is better as the apostrophe is not needed. It will vary depending on whether you are writing all caps or not - for the sake of clarity an apostrophe may be needed.
The general consensus is that you should never use an apostrophe.  But if the addition of an s to an acronym would appear to give the acronym a different meaning, then an apostrophe would be in order.   Acronyms and initialisms are "regular" nouns;  plurals are formed by adding "s".   Since acronyms are capitalized letters, the addition of a small s should not make a difference and it would be clearly understood.
It is generally agreed that an s without an apostrophe is the correct way to pluralise an acronym. There will be some people that argue that apostrophes can be used to pluralise words if they help to ensure correct understanding.