Answer Last Updated: Jul 29, 2016 Views: 2038
According to the Chicago Manual of Style, if the abbreviation is capitalized as in MBAs, do no use an apostrophe. However, if abbreviation is lower case, such as x’s and y’s, the apostrophe is included.
APA Style states, "don’t include an apostrophe when pluralizing abbreviations."
MLA Style also states that apostrophes are not used for plural abbreviations.