Words like CD and 1960 have plurals of CDs and 1960s, although CD's and 1960's were once the recommended way of doing it.
If making a plural with just an s might be confusing (as with single lowercase letters), you can use an apostrophe. So dot your i's and cross your t's. But give people credit: mostly they know a plural when they see one -- there's no need for apostrophes in:
They sent out three SOSs. We have videos for sale. She got five As in her exams. A list of DOs and DON'Ts.
Perhaps the first apostrophe in lower case do's and don'ts is wise.