As a professional typographer for about twenty years, until there was no such thing any longer, I can attest that the apostrophe can and should be used to form the plural of a letter or number. Normally, of course, it is incorrect to use "apostrophe-s" to form a plural; this is the only exception, to my knowledge, to the general rule.

When I began my type-shop career as a proofreader, I was unaware of this exception to the usual rule and marked a usage similar to "two b's in Bobert" as incorrect ~ and subsequently learned that I was wrong.

Strunk and whatzizname's "Elements of Style" probably will verify this; that's the main "rulebook" we used.

This rule (or, more precisely, this exception to a rule) probably came about a matter of clarity: while we might be able to recognize "bs" as the plural of the letter "b" (especially in context), we would more likely be confused when confronted with "as" or "is" (as opposed to "a's" or "i's"), since those plurals-of-vowels can also be seen as two-letter English words.

Of course, all of the above refers to accepted usage of American English. It's an obscure and counterintutive rule, and may not be followed in the UK or elsewhere inthe English-speaking world.