You should almost never use an apostrophe to indicate a plural. Lowercase single letters used as nouns are a rare exception, because omitting the apostrophe can make the sentence much harder to understand (for example, "mind your p's and q's" is much clearer than "mind your ps and qs").

People often misremember this rule and use apostrophes to form plurals for things like single capital letters, abbreviations, numerals, and acronyms/initialisms, but most style guides don't recommend this. So: "CDs", rather than "CD's".

The Chicago Manual of Style says:

Capital letters used as words, numerals used as nouns, and abbreviations usually form the plural by adding s. To aid comprehension, lowercase letters form the plural with an apostrophe and an s.

The lowercase letter exception presumably exists because omitting the apostrophe can make the sentence much harder to understand (for example, "mind your p's and q's" is much clearer than "mind your ps and qs"). I don't know if the rule was different in earlier editions of Chicago, but it's very much in keeping with the guide's philosophy of prioritizing clarity and simplicity over prescriptivist rules.

As a general rule, apostrophes are only used to form plurals if the word would be ambiguous otherwise. In practice, this means that single lowercase letters get an apostrophe plus an S (for example, "mind your p's and q's" is better than "mind your ps and qs"), but capital letters, abbreviations, numerals, and acronyms/initialisms don't. So: "CDs", rather than "CD's".

You should almost never use an apostrophe to indicate a plural. Lowercase single letters used as nouns are a rare exception, because omitting the apostrophe can make the sentence much harder to understand (for example, "mind your p's and q's" is much clearer than "mind your ps and qs").

People often misremember this rule and use apostrophes to form plurals for things like single capital letters, abbreviations, numerals, and acronyms/initialisms, but most style guides don't recommend this. So: "CDs", rather than "CD's".