My own opinion is that an apostrophe here is unnecessary but not unacceptable. Usually, of course, the use of apostrophe s to indicate a plural is an abomination, but some people have a more relaxed attitude to plurals of, for example, numbers and letters, where they think an apostrophe makes the text easier to read and understand. Others may take a more hardline view.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.