Did You Know?
APOSTROPHES AND THE PLURALS OF LETTERS AND NUMBERS
If we simply added an s to make individual letters plural, then letters such as a and i would become, confusingly, as and is. To avoid this confusion, the convention has developed of using an apostrophe to show plurals of lower case letters, for example a's and i's and p's and q's. Nowadays with computers, it is easy to italicise the letters, as in as and is and ps and qs. Do you have a preference?
The following comments on the plurals of numbers may help. It is much easier to make numbers plural because putting an 's' after a number is not confusing: 8s, 9s or 747s. The same is true of decades, centuries, etc: the 1930s or the 1500s. Note, however, that some people still prefer to use an apostrophe for concepts such as 1930’s because what is required is not the plural of the year 1930 but a term to indicate all the years from 1930–1939. Most modern grammar books prefer 1930s. At the end of the day, styles are about being consistent.