With single letters, don't use the the apostrophe. You use them with an abberviation or acronym to show that the "s" is there to make it plural is not part of the original abbrevation/acronum.
Let's say you work with two things, one known at PBCS and the other as PBC. If you wanted to make PBC plural and wrote it as PBCs, your reader could be confused - but if you say PBC's, it's clear. O