Very often one sees an apostrophe (followed by an “s”) used to denote a plural, e.g. pizza’s. Here is a tip for those who don’t want to learn the rules:
Don’t use an apostrophe again and you will be correct more often than not.
I worked with style guides that recommend not using apostrophes for plurals and am very comfortable not to waste an extra keystroke.
Acronyms and All Caps
Many good schools actually teach the rule that acronyms, initialisms, abbreviations and contractions should use an apostrophe before the “s”. So you will see examples such as ICBM’s, QANGO’s, etc. This is a style issue and if you work in such a company, you may have to abide by those rules.
When a block of text is in ALL CAPS and includes an acronym, there is a temptation to put an apostrophe to “clarify” that the S is not another letter in the acronym.
The misused apostrophe is often called the Grocer’s apostrophe perhaps because greengrocers in the west may have come from non-English-speaking countries and have poor language skills. Many languages don’t have a plural form for nouns, hence we see signs such as “POTATOE’S”, “CHIP’S”, “PIZZA’S” etc.
Sometimes the writer thinks that “foreign” words should be given the apostrophe treatment, so you might see a menu with headings such as PIZZA’S and DRINKS.
When the Apostrophe is Correct
Acronyms ending in S may get away with an Apostrophe S. e.g. “The sinking ship sent many SOS’s” is clearer than “The sinking ship sent many SOSs” but more so if there was a plaque caption in all caps, e.g. “THIS MORSE KEY WAS USED TO SEND HUNDREDS OF SOS’S BEFORE THE SHIP SANK.